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22
Phylum Echinodermata
“What do they find to study?” Hazel
continued. “They’re just starfish. There’s
millions of ‘em around. I could get you
a million of ‘em.”
John Steinbeck
Cannery Row, 1945
S
ome of the most familiar seashore animals are members of the phylum
Echinodermata (Greek echinos, “spiny”; derma, “skin”). The phylum contains about 7,000 living species, including the sea lilies, feather stars, sea
stars, brittle stars, sea urchins, sand dollars, and sea cucumbers (Figures 22.1, 22.2,
and 22.3). Another 13,000 or so species are known from a rich fossil record dating
back at least to early Cambrian times.
Echinoderms range in size from tiny sea cucumbers and brittle stars smaller
than 1 cm, to sea stars that exceed 1 m in diameter and sea cucumbers that reach
2 m in length. Except for a few brackish-water forms, echinoderms are strictly marine. They have been prevented from invading land or fresh water, presumably, by
their cutaneous gas exchange methods and their lack of excretory–osmoregulatory structures. In the sea, however, they are widely distributed in all oceans and at
all depths. With the exception of a few odd pelagic sea cucumbers (Figure 22.1P,Q)
and one (Rynkatropa pawsoni) that is commensal on deep-sea anglerfish, all echinoderms are benthic. Some play important roles in marine ecosystems as high-level
predators (certain sea stars) or algal grazers (many sea urchins). In some regions of
the deep sea they may compose 95 percent of the biomass.
Echinoderms are deuterostomes, and their development is frequently cited as
stereotypical of that assemblage. With a few exceptions, living echinoderms possess a well developed coelom, an endoskeleton composed of unique calcareous ossicles, and pentamerous radial symmetry. They are the only fundamentally pentamerous organisms in the animal kingdom. However, this symmetry is
secondarily derived, both evolutionarily and developmentally, and the larval
forms are always bilateral. Among other defining characteristics (Box 22A) is a
uniquely echinoderm feature known as the water vascular system, a complex system of channels and reservoirs that is derived from the coelom and serves a variety of functions.
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CHAPTER TWENTY-TWO
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(A)
(B)
(C)
(D)
(E)
(F)
(G)
G may be replaced by photo
(I)
(H)
(J)
(K)
PHYLUM ECHINODERMATA
3
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(L)
(M)
Electronic file
(N)
(O)
(P)
(Q)
Figure 22.1 Representative echinoderms. (A) Crinoids
from the California coast (Crinoidea). (B) Linckia laevigata.
(C) Astropecten armatus (class Asteroidea). (D) Pteraster tesselatus (Asteroidea). (E) Odontaster crassus (Asteroidea). (F)
Acanthaster planci, the Indo-West Pacific crown-of-thorns
(Asteroidea). (G) The “sea daisy” Xyloplax medusiformis
(Asteroidea). (H) A brittle star, Ophiopholis aculeata (Ophiuroidea). (I) A basket star (Ophiuroidea). (J) Strongylocentrotus purpuratus, the common pacific sea urchin (Echinoidea).
(K) Dendraster excentricus, a sand dollar (Echinoidea). (L) An
“irregular” sea urchin, Lovenia (Echinoidea). (M) The sea
cucumber Parastichopus (Holothuroidea). (N) The strange
deep-sea holothurian Scotoplanes, which lacks podia on the
“dorsal” surface (Holothuroidea). (O) Euapta (Holothuroidea). (P) A pelagic holothurian, Palagothuria (Holothuroidea).
(Q) An epibenthic swimming holothurian, Enypniastes
(Holothuroidea), photographed at 1,586 meters.
Taxonomic History and
Classification
Echinoderms have been known since ancient times;
their likenesses appear in 4,000-year-old frescoes of
Crete. Jacob Klein is credited with coining the name
Echinodermata in about 1734 in reference to sea urchins.
Linnaeus placed the echinoderms in his taxon Mollusca,
along with a mixed bag of other invertebrates. For nearly a hundred years these animals were allied with various other groups, including the cnidarians in Lamarck’s
Radiata. It was not until 1847 that Frey and Leukart recognized the echinoderms as a distinct taxon.
Since the middle of the nineteenth century controversies have centered on classification within the phylum,
and arguments continue today. The abundant fossil
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CHAPTER TWENTY-TWO
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record has been both a blessing and a burden because
authors have treated the fossil evidence in different
ways. Some emphasize differences between morphological types and assign higher categorical ranks to
nearly every fossil taxon discovered; consequently, certain schemes recognize as many as 25 separate classes of
echinoderms. Others apply the evidence more parsimoniously, seeking to establish fundamental similarities;
their schemes recognize fewer classes.
In 1986 Baker et al. established a new class (the
Concentricycloidea) to accommodate a strange deep-sea
echinoderm discovered in association with bacteria-rich
sunken wood. This creature, named Xyloplax medusiformis, and a second species, X. turnerae (Rowe et al.
1988), are now widely viewed as highly modified asteroids (Smith 1988).
The classification scheme below draws from various
authors. It recognizes five classes to which the living
echinoderms belong, but we introduce some of the important fossil forms in the phylogeny section at the end
of the chapter. The reader is cautioned that other classification schemes exist for the taxa within the classes
treated here.
PHYLUM ECHINODERMATA
CLASS CRINOIDEA: Sea lilies and feather stars (Figures 22.1A,
22.3A,B). Body form as a cup or calyx, with oral surface directed upward; aboral stalk, when present, arising from calyx;
ambulacra on arms that bear pinnules; ambulacra may branch
more than once, branches equal; ambulacral grooves open;
skeletal plates fused in calyx, but articulated elsewhere; no external madreporite; mouth and anus on oral surface. About
625 living species (e.g., Antedon, Asterometra, Comantheria,
Comanthina, Isometra, Metacrinus, Neometra, Phixometra, Zygometra).
CLASS ASTEROIDEA: Sea stars (Figures 22.1B–F, 22.3C).
Body stellate with five or more arms; arms not set off from central disc by distinct articulations; anus on aboral surface; mouth
directed toward substratum; ambulacral grooves open; tube
feet with internal ampullae, with or without suckers; madreporite aboral on CD interambulacrum. About 1,500 extant
species. The classification below is a conservative one (for an
alternative scheme, see Blake 1987).
ORDER PLATYSTERIDA: Considered by some to
include most primitive asteroids; tube feet lack suckers; anus absent. Generally restricted to soft substrata. This order has been abandoned by some specialists. Living species are confined to two genera:
Luidia (about 60 species), and Platysterias (monotypic, P. latiradiata).
ORDER PAXILLOSIDA: Upper surface with umbrella-like clusters of ossicles called paxillae; tube feet
lack suckers; anus present or absent. Epibenthic or
shallow burrowers (e.g., Astropecten, Caymanostella, Ctenodiscus, Lethmaster).
ORDER VALVATIDA: Tube feet with suckers; anus
present; some possess paxillae. Widely distributed,
with several hundred species (e.g., Amphiaster, Archaster, Asterodon, Chaetaster, Hoplaster, Linckia,
Odonaster, Oreaster).
Figure 22.2 Schematic sections of the six living classes
of echinoderms, showing body orientations to the substratum and disposition of the ambulacral surfaces.
ORDER SPINULOSIDA: With 5–18 arms; tube feet
with suckers; anus present; generally lacking pedicellariae. With a few hundred species (e.g., Acanthaster, Echinaster, Henricia, Pteraster, Remaster, Solaster).
ORDER FORCIPULATIDA: With 5–50 arms; tube
feet with suckers; anus present; with pincer-like
pedicellariae. Widely distributed sea stars, including
most intertidal forms. Several hundred species (e.g.,
Asterias, Brisinga, Evasterias, Heliaster, Leptasterias,
Pisaster, Pycnopodia, Stylasterias).
“SEA DAISIES”: Body discoidal (< 1 cm diameter);
with ring of marginal spines, but without arms or
rays; skeletal plates arranged concentrically; suckerless podia in a ring near body margin; two ring
canals with hydropore on CD interambulacrum; five
large ossicles on aboral surface mark ambulacra; gut
absent or incomplete. Classification of the enigmatic sea daisies (previously the class Concentricycloidea) is problematic, but many authorities assign
them to the Spinulosida.
CLASS OPHIUROIDEA: Brittle stars and basket stars (Figures
22.1H,I, 22.3F). Body with five unbranched or branched articulated arms, clearly set off from a central disc; ambulacral
grooves closed; coelom in arms greatly reduced by presence of
skeletal vertebrae; tube feet with internal ampullae but without
suckers; anus lacking; madreporite on CD interambulacral
plate on oral surface, often reduced. About 2,000 extant
species in three orders.
ORDER OEGOPHIURIDA: Without bursae; arms
lack dorsal and ventral shields; madreporite on edge
PHYLUM ECHINODERMATA
BOX 22A
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Characteristics of the
Phylum Echinodermata
1. Calcareous endoskeleton arising from mesdermal
tissue and composed of separate plates or ossicles; each plate originates as a single calcite crystal and develops as an open meshwork structure
called a stereom, the interstices of which are filled
with living tissue (the stroma)
2. Adults with basic pentamerous radial symmetry
derived from bilaterally symmetrical larvae (when
present); body parts organized about an
oral–aboral axis
3. Coelomic water vascular system composed of a
complex series of fluid-filled canals, usually evident externally as muscular podia
4. Embryogeny fundamentally deuterostomous,
with radial cleavage, entodermally derived mesoderm, enterocoely, and mouth not derived from
the blastopore
5. Gut complete except where secondarily incomplete or lost
6. No excretory organs
7. Circulatory structures, when present, compose a
hemal system derived from coelomic cavities and
sinuses
8. Nervous system diffuse, decentralized, usually
consisting of a nerve net, nerve ring, and radial
nerves
9. Mostly dioecious; development direct or indirect
of disc; digestive glands extend into proximal portions of arms. A single living species (Ophiocanops
fugiens).
ORDER PHRYNOPHIURIDA: Bursae present; ventral arm shields rudimentary, dorsal shields usually
absent; arms branched or unbranched, but can coil
vertically; madreporite on oral surface; digestive
glands confined to central disc. Includes some primitive brittle stars and the basket stars (e.g., Asteronyx,
Astrodia, Gorgonocephalus, Ophiomyxa).
ORDER OPHIURIDA: Bursae present; dorsal and
ventral arm shields present and usually well developed; unbranched arms incapable of coiling vertically; madreporite on oral surface; digestive glands
wholly within central disc. Includes vast majority of
living brittle stars (e.g., Amphiophiura, Amphipholis,
Amphiura, Ophiactis, Ophiocoma, Ophioderma,
Ophiolepis, Ophiomusium, Ophionereis, Ophiopholis,
Ophiothrix, Ophiura).
CLASS ECHINOIDEA: Urchins and sand dollars (Figures
22.1J–L, 22.3G,I). Body globose or discoidal, often secondarily bilateral; skeletal plates joined by collagen matrix and calcite interdigitations as solid test; with movable spines; water
canals within test; ambulacral grooves closed; with internal jaw
apparatus (Aristotle’s lantern). About 950 extant species in two
extant subclasses (for a more detailed version of the classification outlined below see Smith 1984).
SUBCLASS CIDAROIDEA: Pencil urchins. Test globular,
ambulacral plates simple, each with a pair of perforations
serving one tube foot; spines large, pencil-like, without epidermal covering; anus at aboral pole; dermal gills absent;
mostly extinct; often considered primitive in the class.
About 140 surviving species in one order (Cidaroida) (e.g.,
Cidaris, Eucidaris, Phyllacanthus, Psychocidaris).
SUBCLASS EUECHINOIDEA: Sea urchins, heart urchins,
lamp urchins, sea biscuits, sand dollars. Test globular or discoidal; numbers of tube feet and spines per plate vary; anal
position varies from aboral to “posterior.” Aristotle’s lantern
variable, absent in heart and lamp urchins. About 800 living
species.
INFRACLASS ECHINOTHURIOIDEA: Test up to 30 cm
in diameter, with large amounts of collagen; deep-water
(1,000–4,000 m) species with very thin, flexible tests that
collapse when removed from water; long, club-shaped
oral spines support body off substratum; anus aboral.
One order (Echinothurioida) with three families. (e.g.,
Araeosoma, Asthenosoma, Phormosoma, Sperosoma).
INFRACLASS ACROECHINOIDEA: Includes all of the
commonly encountered urchins and sand dollars; divided into three extant cohorts.
COHORT DIADEMATACEA: Hollow-spined “regular” sea urchins. Anus aboral; with compound ambulacral plates; spines hollow. Three orders, each with
one extant family (e.g., Astropyga, Aspidodiadema,
Caenopedina, Diadema, Micropyga, Plesiodiadema).
COHORT ECHINACEA: Solid-spined “regular” sea
urchins. Anus aboral; with compound ambulacral
plates; spines solid; with five pairs of gills arranged in
circle on peristomial membrane. Three extant orders
(e.g., Arbacia, Echinometra, Echinus, Heterocentrotus,
Paracentrotus, Salenia, Strongylocentrotus, Toxopneustes, Tripneustes).
COHORT IRREGULARIA: Heart urchins, lamp urchins, “irregular” urchins (sand dollars, sea biscuits,
and their relatives). Body globular or discoidal, with
tendency toward bilateral symmetry; anus variable,
shifted to “posterior” (even oral) position; spines usually tiny, forming dense covering; Aristotle’s lantern
reduced, absent in heart and lamp urchins. Perhaps
six extant orders, including the Clypeasteroida (sand
dollars) and several groups of urchins (e.g., Cassidulus, Clypeaster, Dendraster, Echinocardium, Echinodiscus, Echinolampus, Encope, Fibularia, Lovenia, Maretia,
Mellita, Meoma, Metalia, Micropetalon, Spatanga,
Urechinus).
CLASS HOLOTHUROIDEA: Sea cucumbers (Figures 22.1M–Q,
22.3J,K). Body fleshy, sausage-shaped, elongate on oral–aboral axis; skeleton usually reduced to isolated ossicles; symmetry pentamerous or secondarily modified by loss of “dorsal”
(bivium) tube feet along ambulacra C and D; tube feet sometimes entirely absent; madreporite internal; ambulacral
grooves closed; with circlet of feeding tentacles around
mouth. About 1,150 extant species in three subclasses.
SUBCLASS DENDROCHIROTACEA: With 8–30 oral tentacles ranging from digitiform to highly branched; tentacles
and oral region with retractor muscles; tube feet present,
but location varies.
ORDER DACTYLOCHIROTIDA: Body often Ushaped and enclosed in flexible test of skeletal
plates; tentacles unbranched; most are deep-water
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burrowers (e.g., Echinocucumis, Mitsukuriella, Rhopalodina, Sphaerothuria, Vaneyella, Ypsilothuria).
ORDER DENDROCHIROTIDA: Body not Ushaped, but is partially enclosed in plates in certain
genera (e.g., Psolus); feeding tentacles typically
branched. Includes many common intertidal cucumbers (e.g., Cucumaria, Eupentacta, Paracucumis,
Placothuria, Psolus, Thyone).
SUBCLASS ASPIDOCHIROTACEA: With 10–30 leaflike or
shieldlike oral tentacles; oral region lacks retractor muscles;
tube feet present.
ORDER ASPIDOCHIROTIDA: Tentacles shieldlike;
respiratory trees present. Includes the largest
holothurians (up to 2 m) (e.g., Actinopygia, Astichopus, Bathyplotes, Holothuria, Isostichopus, Parastichopus, Stichopus).
ORDER ELASIPODIDA: Typically deep-sea cucumbers, often with strange body forms; respiratory
trees absent (e.g., Benthodytes, Deima, Enypniastes,
Pelagothuria, Scotoplanes).
SUBCLASS APODACEA: With up to 25 tentacles; tentacles
vary from digitate to pinnate; tube feet highly reduced or
absent.
ORDER MOLPADIDA: Body stout, narrowed posteriorly to a distinct tail; with 15 digitate tentacles;
lacking tube feet (e.g., Caudina, Molpadia, Trochoderma).
ORDER APODIDA: Body vermiform; lacking tube
feet; with 10–25 tentacles. Among the apodids is
the bizarre family Synaptidae, with unique anchor
ossicles that occur in densities up to 1,500/sq cm
and provide gripping power (in lieu of tube feet) by
protruding and retracting into the skin in peristaltic
waves along the cucumber’s body wall (e.g., Euapta, Leptosynapta, Synapta).
The Echinoderm Bauplan
The success of the echinoderm bauplan lies partly in the
exploitation of radial symmetry imposed upon a relatively “advanced” coelomate architecture, including a
mesodermally derived calcareous endoskeleton. We
have seen the tendency among radially symmetrical animals to be either sessile or planktonic and to face their
environments on all sides as suspension feeders or passive predators. This generalization applies not only to
those creatures with primary radial symmetry (e.g.,
cnidarians), but also to many of those that have secondarily become functionally radial by way of a sessile
lifestyle (e.g., tube-dwelling polychaetes, entoprocts, ectoprocts, phoronids, and others). Ehinoderms, on the
other hand, have uniquely combined mobility with radial symmetry, and they display a host of feeding strategies and lifestyles. Like other radially arranged animals,
the echinoderms have a noncentralized nervous system,
a feature that allows most of them to engage their environments equally from all sides.
Much of the biology of echinoderms is associated
with their unique water vascular system (see Figure
22.5),which is derived largely from specialized parts of
the left mesocoelic portion of their tripartite coelom. The
water vascular system is a complex of fluid-filled canals
and reservoirs that aid in internal transport and hydraulically operate fleshy projections called tube feet.
The external parts of the tube feet, or podia, can serve a
variety of functions, including locomotion, gas exchange, feeding, attachment, and sensory reception.
These versatile structures have contributed greatly to
the success of echinoderms.
Although modern echinoderms are basically pentaradial creatures, several secondarily derived conditions
exist. In the general case, five sets of body parts are oriented about a central disc. Extending from the mouth at
the center of the oral surface are rows of podia associated
with ambulacral grooves (Figure 22.2), which define
body radii called ambulacra. A radius bisecting adjacent
ambulacra is called an interambulacrum. In a sea star,
for example, the ambulacra are represented by the arms,
and the interambulacra by the areas between the arms.
In many echinoderms (e.g., ophiuroids, holothurians,
and echinoids), the ambulacra are not marked by wide
or “open” external furrows, in which case the animals
are said to have “closed” ambulacral grooves. The side(s)
of the body on which tube feet occur are often referred to
as the ambulacral surface(s).
The pentaradial symmetry of modern echinoderms is
thought to have evolved from a triradiate (adult) plan;
such a condition occurs in an extinct group called the
helicoplacoids (see Figure 22.19B). Although it may not
be immediately obvious, the pentamerism of all echinoderms can be described in terms of reference to particular radii. When present externally, the position of the
opening to the water vascular system (the madreporite)
gives a clue to body orientation because it lies on a particular interambulacrum. A system of lettering has been
developed in which the ambulacrum opposite the
madreporite is coded A; the others are then coded B
through E in a counterclockwise fashion as viewed from
the aboral surface (Figure 22.3C). Thus, the madreporite
Figure 22.3 External anatomy of echinoderms. (A)
Botryocrinus, a stalked fossil crinoid. (B) Neometra, a 30armed, nonstalked crinoid. (C) Aboral view of Ctenodiscus
(Asteroidea). The ambulacral radii are labeled according to
convention. (D,E) Aboral and oral views of Xyloplax (the
sea daisy). (F) The ophiuroid Asteronyx crawling on a gorgonian. Note the highly articulated arms. (G) The sand
dollar Dendraster (aboral view). Note the petaloids through
which the respiratory podia extend. (H) Oral view of the
sand dollar Encope (Echinoidea). (I) The sea urchin
Plesiodiadema has extremely long spines and podia. (J)
Cucumaria planci, a dendrochirotacean sea cucumber. (K)
The highly modified pelagic holothurian, Pelagothuria.
PHYLUM ECHINODERMATA
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lies between ambulacra C and D (i.e., on the CD interambulacrum). Radii C and D are said to compose the
bivium, while radii A, B, and E compose the trivium.
As we explore the phylum in more detail, keep these
generalities in mind and think of echinoderm diversity
as variations on this pentamerous theme.
Body Wall and Coelom
An epidermis covers the bodies of all echinoderms and
overlies a mesodermally derived dermis, which contains the skeletal elements, called ossicles (Figure
22.4A–D). Internal to the dermis and ossicles are muscle
fibers or layers and the peritoneum of the coelom. The
degree of development of the skeleton and muscles
varies greatly among groups. In urchins and sand dollars, the ossicles are firmly attached to one another to
form a rigid test, and the body wall muscles are weakly
developed. In sea cucumbers, however, the ossicles are
separate and lie scattered in the fleshy dermis (Figure
22.4D); here distinct muscle layers are present. Between
these extreme conditions are cases in which adjacent
skeletal plates articulate to various degrees. In the arms
of sea stars and brittle stars, for example, the body wall
muscles are arranged in bands between the plates, providing various degrees of arm motion. In some groups
the skeletal plates are developed to such a degree that
they nearly obliterate internal cavities. In brittle stars,
for example, each arm “segment” contains a central
skeletal ossicle called a vertebra (see Figure 22.9A,B),
and the arm coeloms are reduced to small channels.
Similarly, the arm coeloms in crinoids are greatly reduced by skeletal plates.
The endoskeleton is calcareous, mostly CaCO3 in the
form of calcite, with small amounts of MgCO3 added.
Developmentally, the skeleton of echinoderms begins as
numerous separate spicule-like elements, each behaving
as a single calcite crystal. Additional material is deposited on these crystals in various amounts, depending on
the ultimate condition of the skeleton. Each ossicle is
porous, has an internal meshwork (the stereom) of lattice-like or labyrinth-like spaces (Figure 22.4D), and
generally is filled with dermal cells and fibers (the stroma). This structure is unique to members of the phylum
Echinodermata.
During the formation of the skeleton, the plates may
remain single (simple plates) or they may fuse to form
compound plates. In addition, they frequently give rise
to bumps and knobs called tubercles, to granules, and
to various sorts of movable and fixed spines (Figure
22.4A,E). In some groups, especially the asteroids and
echinoids, the skeleton also produces unique pincer-like
structures called pedicellariae (Figure 22.4E–I). These
structures respond to external stimuli independently of
the main nervous system, and they possess their own
neuromuscular reflex components. Pedicellariae were
discovered in 1778 by O. F. Müller, who described them
as parasitic polyps and gave them the generic name
Figure 22.4 Structure of the echinoderm body wall and
some skeletal elements. (A) The body wall of an urchin
(composite section). (B) Spines on the sand dollar
Echinarachnius parma (SEM). The arrows point to ciliary
tracts. Scale bar represents 100 µm. (C) Skeletal ossicles
from the central discs of four species of brittle stars
(Ophiuroidea), shown in top (top row), side (middle row),
and basal (bottom row) views. Scale bar represents
0.05 mm. (D) Skeletal ossicles from the holothurian Psolus
chintinoides. The stereom structure is shown at two magnifications. Scale bar represents 100 µm. (E) Types of echinoid pedicellariae surrounding the base of a large spine.
(F,G) Elevated pedicellariae used for prey capture by the
sea star Stylasterias forreri: F, pedicellariae open and
extended; G, pedicellariae retracted. (H) Details of a generalized pedicellaria. (I) Two types of muscle systems in
pedicellariae. (J) A movable spine (section). Note the position of the muscles relative to the body wall layers.
Pedicellaria. He recorded three species of these “parasites” (P. globifera, P. triphylla, and P. tridens); forms of
these names are still used to describe different types of
pedicellariae.
Nearly a century after Müller’s discovery, it was realized that pedicellariae are actually produced by the
echinoderms themselves, but their exact nature remained elusive. Louis Agassiz believed they were the
young of the animals on which they occurred. Even
today there are competing opinions about their functions (see Campbell 1983 for a review). Pedicellariae differ not only in their structural details, but in their size
and distribution on the body. Some are elevated on
PHYLUM ECHINODERMATA
UNCORRECTED PAGE PROOFS
(B)
(C)
(D)
(E)
(F)
(G)
Open
pedicellariae
(H)
(I)
(J)
9
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CHAPTER TWENTY-TWO
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stalks, whereas others lie nestled directly on the body
surface, either singly or in clusters. Some help keep debris and settling larvae off the body, and others are used
to defend against larger organisms. The sea urchin
Toxopneustes bears toxin-producing pedicellariae with
which it discourages would-be predators. In some
urchins the pincers grasp and hold objects for camouflage and protection. A few sea stars actually use their
pedicellariae to capture prey (Figure 22.4F,G).
Movable spines and pedicellariae contain muscles
and other tissues that lie outside the main skeletal
framework of the body wall (Figure 22.4H–J). This
arrangement raises some interesting questions concerning the method of nutrient supply to these tissues because they are isolated from the coelom and gut.
Pedicellariae may absorb nutrients directly from the
water, or they may actually trap and digest small organisms and then absorb the products (Stephens 1968;
Pequignat 1966, 1970; Ferguson 1970).
As in all deuterostomes (except the Chordata), the
coelomic system of echinoderms usually develops as a
tripartite series, originating as paired proto-, meso-, and
metacoels. However, with the transformation to radial
symmetry, these coelomic cavities do not come to lie in
the three body regions usually associated with deuterostome bauplans. The main body coeloms are derived
from the embryonic metacoels and are well developed
in most groups. Other coelomic derivatives include the
water vascular system, gonadal linings, and certain
neural sinuses.
The main body cavities, or perivisceral coeloms, are
lined with ciliated peritoneum, and their coelomic fluid
plays a major circulatory role. A variety of coelomocytes
are present in the body fluid and in the water vascular
system. Many of these cells are phagocytic. Hemoglobin
occurs in the coelomocytes of many holothurians and a
few brittle stars.
Water Vascular System
The water vascular system is intimately involved in
many aspects of echinoderm biology, and a discussion
of its anatomy is a necessary preface to other considerations. It is perhaps easiest to begin with an examination
of the system in a sea star and then treat the other taxa.
Asteroidea. Figure 22.5A is a schematic representation of the water vascular system of a sea star. The system opens to the exterior through a special skeletal
plate, the madreporite, or sieve plate, located off-center on the aboral surface on the CD interambulacrum
(Figure 22.3C). The madreporite is perforated and
deeply furrowed, and the overlying epidermis is ciliated and porous where it lines the furrows. The function of the madreporite has been the subject of much
controversy. The traditional view that it serves as an
avenue for sea water to enter the system has been challenged because the fluid in the system differs from sea
water. However, using radioactive tracers, Ferguson
(1984) demonstrated that water does in fact enter
through the madreporite. We still lack a clear understanding of how this structure functions.
Internally, the madreporite forms a cuplike depression, the lumen of which is called the ampulla, that
communicates with other coelomic derivatives of the
water vascular system and the hemal system (discussed
below). From the lower end of the ampulla arises the
stone canal, so named because of the skeletal deposits
in its wall. A portion of the hemal system called the axial
sinus (discussed below) is often intimately associated
with the stone canal. The stone canal descends orally
and joins with a circular ring or circumoral canal,
which extends around the central disc on a plane perpendicular to the body axis. In addition to a radial canal
extending into each arm, the ring canal gives rise to
blind pouches called Tiedemann’s bodies and polian
vesicles (Figure 22.5A,B). There is some uncertainty
about the functions of these pouches, but it is suspected
that the former produce certain coelomocytes and the
latter help regulate internal pressure within the water
vascular system.
The fluid in the water vascular system is similar to
sea water, but it includes various coelomocytes, certain
organic compounds such as proteins, and a relatively
high concentration of potassium ions. This fluid is
moved through the system largely by the action of cilia
that line the canal epithelium. Some of the canals, especially the stone and ring canals, contain internal partition-like extensions of their inner walls that probably
help direct the flow of fluid. Ferguson and Walker
(1991) describe the stone canal in some sea stars as a
“ciliary pump” that draws fluid into the water vascular
system from both the madreporite and the axial sinus of
the hemal system. Thus, it appears that the liquid in the
water vascular system is a combination of environmental sea water and body fluid.
In each arm the radial canal gives rise to numerous
lateral canals, each of which terminates in a tube foot. In
most asteroids, each tube foot consists of a bulbous ampulla and a hollow, muscular, suckered podium (Figure
22.5B). Members of the orders Platyasterida and
Paxillosida lack suckers on their tube feet. The ampullae
are internal and lie above the skeletal plates of the ambulacral groove. The podia extend to the outside and
contain the usual body wall muscle layers around a
coelomic lumen and sometimes include supportive ossicles. In asteroids the tube feet serve primarily for locomotion and temporary attachment, and to hold prey
during feeding. In addition, they are usually highly
touch-sensitive. At the tip of each radial canal is an unsuckered, tentacle-like, sensory terminal tube foot.
The operation of the tube feet depends on hydraulic
pressure regulation and on muscle action of the individual ampullae and podia. Fluid is supplied to each podium from the main canal system. The ampulla acts as a
PHYLUM ECHINODERMATA
(A)
11
UNCORRECTED PAGE PROOFS
Figure 22.5 The water vascular system and related
structures. (A) General structure of the water vascular system in an asteroid. (B) A sea star arm (cross section). (C)
The central disc (oral view) of an ophiuroid (Amphiura).
The madreporite is on the CD interambulacrum. (D) The
end of a crinoid pinnule (longitudinal section). The podia
occur as clusters. (E) The cleaned test (aboral view) of the
sea urchin Echinus. The madreporite is on the CD interambulacrum. (F) The periproct and surrounding plates of
Strongylocentrotus. (G) The water vascular system of
Xyloplax.
(B)
(C)
(D)
(E)
(F)
(G)
reservoir for fluid used to operate its associated podium. A valve in the lateral canal can effectively isolate the
tube foot from the rest of the system. When the ampulla
is filled with fluid and the lateral canal valve is closed,
the ampulla contracts and forces fluid into the podium.
The sucker is then pressed against the substratum and
12
CHAPTER TWENTY-TWO
UNCORRECTED PAGE PROOFS
held there by adhesive secretions of the epidermis. Next
the longitudinal muscles of the podium contract; this action shortens the tube foot and forces the fluid back into
the now relaxed ampulla. At the same time, other muscles raise the center of the sucker disc and create a vacuum, like that of a suction cup. Release of the sucker involves relaxation of the podial muscles and contraction
of the ampulla; this action again forces fluid into the
lumen of the podium and releases the suction. In addition to this attachment–detachment action, the podia are
also capable of bending by differential contraction of the
longitudinal muscles.
The water vascular system of the sea daisies
(Xyloplax) is unique among the echinoderms (Baker et
al. 1986). A madreporite homologue, the hydropore,
opens on the aboral surface on the CD interambulacrum
(Figure 22.3D) and connects internally to a pair of concentric water canals (Figure 22.5G). Polian vesicles lie on
the other four interambulacra. The outer, marginal
water canal gives rise to peripherally located suckerless
podia. Each podium bears an internal ampulla. This is
the only echinoderm water vascular system in which
the podia are not arranged along the ambulacra.
Ophiuroidea. The water vascular system of brittle
stars is similar to that of asteroids. However, the
madreporite is on the oral surface of the central disc,
on the CD interambulacrum, and the internal plumbing is modified accordingly (Figure 22.5C). In some
ophiuroids (e.g., Ophioderma appressun) the madreporite is reduced to two tiny pores. Apparently, most
of the fluid in this type of system is drawn from the
axial sinus by the stone canal (Ferguson 1995).
The ring canal bears polian vesicles, but apparently
lacks Tiedemann’s bodies. The ring canal gives off the
usual five radial canals and also branches to a wreath of
buccal tube feet around the mouth. In basket stars the
arms and the radial canals are branched. The suckerless
podia are highly flexible, finger-like structures that secrete copious amounts of sticky mucus. They function
primarily as feeding, digging, and sensory organs.
Crinoidea. The water vascular system of crinoids
operates entirely on coelomic fluid. There is no external madreporite; rather, a number of “stone canals”
arise from the ring canal and open to coelomic channels. Some species possess hundreds of such stone
canals. The main perivisceral coeloms bear ciliated
funnels to the exterior through which water enters the
body cavities, perhaps as an indirect method of regulating hydraulic pressure in the water vascular system.
From the ring canal arise the main radial canals that
extend into each arm and paired oral tube feet that appear at each interambulacrum. The number of arms in
crinoids ranges from five to as many as two hundred,
and in many cases the arms are branched. The number
of radial canals corresponds to the arm number in each
species, and they are branched in those with branched
arms. Furthermore, crinoid arms bear tiny side branches called pinnules (Figure 22.3B), into which branches
of the radial canals extend. Suckerless podia occur along
the pinnules, often in clusters of three (Figure 22.5D),
and each cluster is served by a branch of the water vascular system. The podia are highly mobile and usually
bear adhesive papillae on their surfaces; they function
primarily as feeding and sensory organs.
Echinoidea. The water vascular systems of sea
urchins and sand dollars may be viewed as modifications of the asteroid plan. These animals bear a special
set of skeletal plates around the aboral pole; one of
these plates is the CD-interambulacral madreporite
(Figure 22.5E,F). To understand the water vascular
system of sea urchins, it is necessary to realize that the
ambulacra, and thus the rows of podia and their internal plumbing, extend around the sides of the body
(like five longitude lines on a globe) to the upper surface, where they converge toward the aboral pole
(Figures 22.2, 22.5E).
The madreporite of echinoids, like that of asteroids,
leads to an ampulla and then to a stone canal (short in
sand dollars and long in sea urchins), which extends
orally to a ring canal surrounding a complex system of
muscles and plates that comprise the feeding apparatus.
The ring canal gives rise to five radial canals, one beneath each ambulacrum. Each radial canal gives off lateral canals leading to tube feet and terminates in a sensory podium near the aboral pole. Unlike the plates in
other echinoderms, the ambulacral plates of echinoids
have holes in them through which the podia pass to the
outside. The tube feet of echinoids may be suckered or
unsuckered, and they serve a variety of functions, including attachment, locomotion, feeding, and gas exchange.
Holothuroidea. In sea cucumbers the water vascular
system contains the major elements seen in other taxa,
but it is organized to accommodate the elongation of
the body. In most holothurians the madreporite is
internal and opens to the coelom. The madreporite
lies beneath the pharynx in the CD-interambulacral
position and gives rise to a short stone canal. A ring
canal encircles the gut and bears from 1 to 50 polian
vesicles. Five radial canals arise from the ring canal
and give off extensions to the oral tentacles before
extending aborally (“posteriorly”) beneath closed
ambulacral grooves. In those species that retain clear
pentamerous symmetry, each radial canal gives rise to
rows of ampullae and suckered podia. In some
species the podia of the bivium (the “dorsal” or upper
surface) are reduced or lost, and in the apodaceans all
of the tube feet are greatly reduced or absent. The
podia of holothurians serve in locomotion and attachment, and are touch-sensitive.
PHYLUM ECHINODERMATA
13
UNCORRECTED PAGE PROOFS
(E)
Support and Locomotion
Except for the holothurians, the general body shape and
structural support of echinoderms are maintained primarily by the skeletal elements. Particular structures,
such as podia and gills, are supported mostly by hydrostatic pressure. In most sea cucumbers, in which the
skeletal plates are usually tiny separate ossicles, the
body wall muscles form thick sheets, adding structural
integrity to the body by working on the coelomic spaces
to provide a hydrostatic skeleton.
Many echinoderms possess certain connective tissues that contribute to body “tone” through rapid
changes in their mechanical properties (Motokawa
1984). In a matter of seconds or minutes the fibers of
these tissues can become relatively rigid, thereby reducing body flexibility. This transformation appears to be
under direct nervous control but does not involve muscular activity.
Locomotor methods among echinoderms are determined by overall body configuration, the animals’
habits, and the nature of the skeletal, muscular, and
water vascular systems. Apart from the sessile sea lilies
(e.g., Ptilocrinus), most extant crinoids are capable of
crawling and swimming, both of which are done with
the oral side directed away from the substratum (Figure
22.6A,B). The aboral cirri are used primarily for temporary attachment and for righting the animal if overturned.
During crawling, the arms are bent downward and
used to lift the body off the substratum; the animal then
walks on its arm tips. Swimming is accomplished by
up-and-down sweeps of the arms, which are divided
into functional sets that move alternately. For example,
in ten-armed species, five arms move upward while the
Figure 22.6 (A) The crinoid, Antedon in a resting position. (B) Antedon as it might appear walking on its arm
tips. (C) A sea star arm (side view) with tube feet in
motion. (D) Changes in position of an individual podium
as the animal moves in the direction of the arrow. The
podium executes its power stroke while in contact with
the substratum (x), and its recovery stroke while lifted
from the substratum. Note the changes in podium length
and the corresponding changes in volume of the ampulla.
(E) The sea star Pisaster giganteus crawling over an irregular substratum.
other five arms move downward. As any given arm is
moving one way, its two neighboring arms are moving
the opposite way. In animals with more arms (usually
multiples of five), the arms are divided into functional
sets of five.
Asteroids exemplify locomotion using podia. The action of a single podium involves power and recovery
strokes, with the process following the same fundamental mechanical principles we have seen in the appendages of many other invertebrates. The sea star’s
arms are held more or less stationary relative to the central disc, even in species with a flexible skeletal framework (e.g., Pycnopodia), and movement is accomplished
by the thousands of podia on the oral surface. Overall
movement is generally smooth because of the high
number of podia and the fact that at any given moment
they are in different phases of the power and recovery
strokes (Figure 22.6C). Although there is some coordination of the action of the tube feet to produce movement
in a particular direction, there are no metachronal waves
14
CHAPTER TWENTY-TWO
UNCORRECTED PAGE PROOFS
of podial motion as seen in many other “multilegged”
creatures. In fact, control of podial action is not fully understood (even isolated arms crawl about normally).
Most sea stars move very slowly, but a few (e.g.,
Pycnopodia) are relative speedsters. Some asteroids that
are usually rather sedentary become extraordinarily
rapid “runners” upon encountering a potential predator
(often another sea star). Some species that cannot escape
by fast movement have evolved other defense mechanisms. The slow-moving Pacific “cushion star,” Pteraster
tesselatus (Figure 22.1D), secretes copious amounts of
mucus, which serves to discourage predators such as
Solaster and Pycnopodia.
If one can follow the action of a single podium during movement (not an easy assignment), the locomotory
forces can be understood (Figure 22.6D). At the end of a
recovery stroke, the podium extends in the direction of
movement and attaches to the substratum. The sucker
remains attached during the power stroke as the longitudinal muscles in the wall of the podium begin to contract, thereby shortening the podium and pulling the
body forward. At the end of the power stroke, the podium lifts from the substratum and swings forward again.
As illustrated, the ability to bend the podia is essential
to the overall action. The huge number of podia and the
general flexibility of the body allows most sea stars to
glide smoothly over even rough and irregular surfaces
(Figure 22.6E).
Ophiuroids use their flexible articulated arms primarily for crawling or clinging (Figure 22.3F). The skeletal arrangement of the arms allows for extensive “lateral” movement on a plane perpendicular to the body
axis, but the arms have almost no flexibility parallel to
the body axis. This feature, coupled with the fragile nature of these animals, causes them to break easily when
lifted by an appendage—hence the common name
“brittle stars.” The tube feet lack suckers and ampullae,
but are equipped with a well developed lattice of muscles in their walls. They are capable of protraction and
retraction and of swinging through arcs. These combined actions of the arms and podia allow many ophiuroids to burrow into soft sediments.
Sea urchins move by the use of podia and movable
spines. Their long suckered podia are capable of a wide
range of motion, and the strong spines provide stiltlike
support and movement. Some “regular” urchins excavate shallow depressions in hard rock. Strongylocentrotus purpuratus, a common West Coast urchin of
North America, forms such pockets in hard substrata,
and members of this species often become trapped in
their self-made homes. These urchins bore largely by the
action of the teeth of their feeding apparatus. Their excavations provide protection in areas of high wave and
surge action.
Some of the irregular urchins burrow well below the
sand surface and maintain an open chimney from their
cavern to the overlying water (see Figure 22.11G). Most
of these soft-sediment burrowers have special spatulate
spines along the sides of the body that aid in digging.
Sand dollars live in or on soft sediments. Some bury
themselves completely, but most keep part of the body
above the surface (see Figure 22.11F). A few, such as
Clypeaster rosaceus, do not burrow at all. Burrowing and
crawling are accomplished largely by the action of movable spines. There has been some controversy about the
function of the deep marginal notches and holes
(lunules) in the tests of some sand dollars (Figure
22.3H). Elegant experiments by Telford (1981, 1983) indicate that these structures help the animals maintain
stability in strong currents. Drag is eased by flow along
surface channels from the center of the body to the
lunules and notches and then away from the test margin. In addition, the lunules reduce the lift generated by
ambient water movements.
Holothurians live on the surfaces of various substrata or else burrow into soft sediments. Crawling is accomplished by the podia or by action of the body wall
muscles. Many epibenthic species are cryptic and usually remain lodged in cracks and crevices or under rocks.
In these forms the podia are used primarily for anchorage and to hold bits of shell and stone against the body
for protection. In a few deep-sea forms (e.g., Scotoplanes;
Figure 22.1N), some of the podia are elongate and used
for walking. In some holothurians (e.g., Psolus), the trivium surface is modified as a creeping, footlike sole. A
few sea cucumbers are pelagic and capable of weak
swimming (Figure 22.1P,Q).
The apodaceans lack locomotor tube feet and most
burrow in sand or mud by means of peristaltic action of
the body wall muscles. Some live completely buried,
whereas others form U-shaped burrows.
Feeding and Digestion
Echinoderms display a great variety of feeding strategies, and we present only a brief survey here. In addition, the structure of the digestive tract differs among
groups, as summarized below.
Crinoidea. Sea lilies and feather stars sit with their
oral sides up and feed by removing suspended material from the surrounding water. The arms and pinnules are usually held outstretched on a plane perpendicular to the ambient water flow, thus presenting
a large food-trapping surface. Many errant forms are
negatively phototactic and emerge from concealment
to feed only at night. Some deep-water species hold
their arms upward and outward, forming a funnel
with which they capture detrital rain.
The open ambulacral grooves extend onto the pinnules and are lined with cilia that beat toward the
mouth. Food particles, including plankton and organic
particulates, contact the podia, which then flick the food
into the grooves (Figure 22.7B). Cilia drive the food to
the mouth, where it is ingested. The primitive nature of
PHYLUM ECHINODERMATA
15
UNCORRECTED PAGE PROOFS
Figure 22.7 Internal anatomy of crinoids. (A) Central
disc and base of one arm (vertical section). (B) An arm
with open ambulacral (food) groove (cross section). (C)
The oral surface of Antedon (cutaway view). The positions
of ambulacral radii are indicated by the letters around the
periphery.
crinoids suggests that this use of the podia and ambulacral grooves for suspension feeding may reflect the
original function of the water vascular system.
The mouth opens to a short esophagus that leads to a
long intestine (Figure 22.7A,C). The intestine loops
around the calyx and then straightens to a short rectum
terminating at the anus, which is borne on an anal cone
near the base of one of the arms. In most species the intestine bears diverticula, some of which are branched.
Although the histology of the crinoid gut has been described, little is known about the digestive physiology
of these animals.
Asteroidea. Most sea stars are opportunistic predators or scavengers. They feed on nearly any dead animal matter and prey on a variety of invertebrates.
Many species are generalists in terms of their food
preferences and may play important roles as highlevel predators in intertidal and subtidal communities. Others are strict specialists. Solaster stimpsoni, a
large northeastern Pacific sea star, feeds exclusively
on holothurians, while a related species (S. dawsoni)
preys on S. stimpsoni!
Among the best known sea stars is the tropical
“crown-of-thorns,” Acanthaster planci. This animal feeds
on coral polyps and has received great notoriety in recent years because of its implication in the destruction
of Indo–West Pacific coral reefs. There is still disagreement concerning the reason for the recent increases in
the size of the Acanthaster populations, but some specialists think that it is a result of human interference in
the predator–prey balance of the reef communities.
Among the major predators of Acanthaster is the giant
triton, Charonia (Gastropoda), which is collected in high
numbers for its handsome shell.
Except for a few suspension feeders (discussed
below), most sea stars depend on an eversible portion of
the stomach to obtain food. Some forms, including
Acanthaster, Culcita (the cushion star), and Asterina (the
bat star), spread the stomach over the surface of a food
source, secrete primary enzymes, and suck in the partially digested soup. In the case of Culcita, the food may
include encrusting sponges or algal mats or organic detritus that has accumulated on the substratum. Asterina
feeds in much the same manner, digesting organic matter under its spread everted stomach. Oreaster extrudes
its stomach over sand, algae, or sea grass and ingests the
associated microorganisms and particulate detritus. It
can, however, switch to a predatory or scavenging
mode when appropriate food sources are encountered.
One Caribbean species (O. reticulatus) feeds primarily
on sponges by everting its stomach and digesting its
prey (Wulff 1995). Many sea stars that feed on large prey
also utilize external digestion by everting the stomach.
Sedentary or sessile prey, such as gastropods, bivalves,
and barnacles, are eaten by a host of asteroid predators,
including the voracious Pacific ochre star Pisaster
ochraceus (Figure 22.8D). This sea star hunches over its
16
CHAPTER TWENTY-TWO
Gonopore
UNCORRECTED PAGE PROOFS
Figure 22.8 Feeding and internal anatomy of
asteroids. (A) The central disc and base of one
arm of a sea star (vertical section). (B) Asterias
(oral view). The mouth is ringed by oral spines
and podia. (C) The internal organs in the central disc and arms of the trivium of Asterias.
Each dissected arm has various organs removed.
(D) A constellation of the predatory sea star
Pisaster ochraceus.
Peristomial
membrane
prey with the oral area pressed
against the potential victim, holding itself in position with its podia.
It then everts the stomach and begins secreting digestive enzymes.
The stomach is very thin and flexible; it can be slid between even the
tightly clamped valves of mussels
and clams, thus liquefying the
prey’s body inside its own shell.
The fluid nutrients are drawn in
with the retracting stomach.
Some sea stars are suspension feeders, consuming
plankton and organic detritus. Henricia, Porania, and a
few others are typically full-time suspension feeders,
and some predatory types, such as Astropecten, are capable of periodic suspension feeding as a means of supplementing their usual diet. In most of these sea stars, particulate food material that contacts the body surface is
trapped by mucus and moved by cilia to the ambulacral
grooves and ultimately to the mouth. Food movement
is by ciliary action. Leptasterias tenera is able to capture
suspended food, such as phytoplankton and small crustaceans, with its pedicellariae and tube feet. The sea star
Novodinia antillensis extends its arms upward into water
currents. The dozen or so arms form a large feeding surface used to capture planktonic crustaceans; the prey are
grasped by pedicellariae. A few species, including
Stylasterias forreri and Labidiaster annulatus, possess
wreathlike circlets of pedicellariae used in prey capture
(D)
PHYLUM ECHINODERMATA
17
UNCORRECTED PAGE PROOFS
(Figure 22.4F,G). These sea stars feed on a variety of animals, even fishes (Chia and Amerongen 1975, Dearborn
et al. 1991).
The digestive system of sea stars extends from the
mouth in the center of the oral surface to the anus in the
center of the aboral surface (Figure 22.8A). The mouth is
surrounded by a leathery peristomial membrane. The
membrane is flexible, allowing eversion of the stomach,
and it contains a sphincter muscle to close the mouth
orifice. Internal to the mouth is a very short esophagus
leading to the cardiac stomach, which is the portion that
is everted during feeding. Radially arranged retractor
muscles serve to pull the stomach back within the body.
Aboral to the cardiac stomach is a flat pyloric stomach,
from which arises a pair of pyloric ducts extending into
each arm. These ducts lead to paired digestive glands,
or pyloric ceca, in each arm (Figure 22.8A,C). A short intestine leads from the pyloric stomach to the anus and
often bears outpocketings called rectal glands or rectal
sacs.
The pyloric ceca and cardiac stomach are the main
sites of enzyme production. These enzymes, mostly proteases, are carried by ciliary action through the everted
stomach and released onto the food material. Digestion
is completed internally, but extracellularly, after ingestion of the liquefied food. Digested products are moved
through the pyloric ducts to the pyloric ceca, where they
are absorbed and stored. The intestine apparently serves
little purpose in the digestive process, but the rectal sacs
are known to pick up nutrients from the intestine, probably salvaging them from potential loss through the
anus.
Many sea stars harbor various commensals that derive their food from scraps of their host’s meals. One
well known relationship is that of a polynoid scale
worm, Arctonoe vittata, and several species of asteroid
hosts, including the Pacific leather star, Dermasterias imbricata. The worm is an obligate symbiont, spending
most of its life cruising and feeding in the host’s ambulacral grooves. Not only is the polychaete chemically attracted to its host, but recent studies indicate that
Dermasterias is also attracted to Arctonoe; this observation suggests that the sea star also may derive some
benefit from the association.
The sea daisy Xyloplax medusiformis lacks a digestive
system, but the oral surface is covered by a membranous velum that may have been derived from the gut
(Baker et al. 1986). These animals may absorb dissolved
organic matter across this velum. Perhaps the source of
the nutrients is bacteria that live in the decomposingwood habitat of these strange asteroids. Xyloplax turnerae has an incomplete gut. A large mouth opens into a
shallow, saclike stomach, but intestine and anus are
lacking.
Ophiuroidea. Brittle stars exhibit a variety of feeding methods, including predation, deposit feeding,
scavenging, and suspension feeding; some species are
capable of more than one method. Some ophiuroids,
such as the basket stars (Figure 22.9F), are really predators that utilize suspension feeding strategies to capture relatively large swimming prey (up to about 3 cm
long).
Selective deposit feeding is accomplished by the
podia and sometimes by the arm spines. The epidermis
of the arms secretes mucus, to which organic material
adheres. The podia roll the mucus and food into a
clump, or food bolus. Near the base of each podium is a
flaplike projection called a tentacular scale (Figure
22.9B,D). The food bolus is transferred from a podium
onto its adjacent scale, picked up by the next podium,
and so on, so that the food is transported along the arm
to the mouth. Suspension feeding by brittle stars usually involves a similar method of transport once food is
trapped. Food capture is sometimes accomplished by
secreting mucous threads among the arm spines and
waving the arms about to trap plankton and organic detritus. The food is moved to the podia and then transported to the mouth. Brittle stars that use this technique
typically have very long arm spines (e.g., Ophiocoma,
Ophiothrix, Ophionereis). Astrosoma agassizii extends its
long (up to 70 cm) arms into the overlying water and
captures planktonic copepods (Ferrari and Dearborn
1989). Some other brittle stars suspension feed by using
extended podia to form a trap; then the podia pass
clumps of food to the mouth (Figure 22.9E).
Predatory suspension feeding by basket stars occurs
mostly at night. At dusk the animals emerge from their
hiding places and assume a feeding position, with their
branched arms held fanlike into the prevailing current,
in a manner similar to the feeding behavior of most
crinoids. Astrophyton muricatum changes position with
the ebb and flow of the tide, always orienting its arms
into the current; it stops feeding at slack tide (Hendler
1982b). When a small animal contacts an arm, the appendage curls to capture the prey. Ingestion is often
postponed until darkness has passed; the prey is then
transferred to the mouth by the flexible arm. These basket stars feed on a variety of invertebrates, such as
swimming crustaceans and demersal polychaetes.
Some brittle stars are active predators, capturing benthic organisms by curling an arm into a loop around the
prey, then pulling it to the mouth. Species that feed in
this manner usually have short arm spines that lie flat
against the arm itself (e.g., Ophioderma). Several species
of brittle stars dig beneath the surface of the substratum
and form semipermanent mucus-lined burrows. The
arms extend to the surface and help maintain ventilation currents within the burrows. Such species are able
to extract food from within the sediment, the substratum surface, and the overlying water (Woodley 1975).
The commensal brittle star Ophiothrix lineata lives in
the atrium of the large sponge Callyspongia vaginalis,
emerging to feed on detritus adhering to its host’s outer
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CHAPTER TWENTY-TWO
UNCORRECTED PAGE PROOFS
Figure 22.9 Feeding and internal anatomy of ophiuroids. (A) The central disc and base of one arm (vertical
section). (B) An ophiuroid arm (cross section). (C) The
central disc of Ophiothrix (oral view). (D) Sequence (1–5)
of movements of a single podium as it passes food toward
the mouth by scraping the podium on an adjacent tentacular scale (Ophionereis fasciata). (E) Podia moving food
bolus toward the mouth in the suspension-feeding ophiuroid Ophiothrix fragilis. (F) The basket star Gorgonocephalus with its arms spread out over a gorgonian to
capture food from the water.
(F)
surface. While keeping the sponge clean, the ophiuroid
is supplied with food and afforded protection from
predators (Hendler 1983).
The digestive tract of ophiuroids is incomplete. The
intestine and anus have been lost, and the digestive system is confined entirely to the central disc (Figure 22.9A).
The mouth leads to a short esophagus and large folded
stomach, which fills most of the interior of the disc and
reduces the coelom to a thin chamber. The stomach is
presumably the site of digestion and absorption.
Echinoidea. Feeding strategies among echinoids
include various kinds of herbivory, suspension feeding, detritivory, and a few forms of predation. In most
“regular” urchins, feeding depends largely on the
action of a complex masticatory apparatus that lies
just inside the mouth and bears five calcareous protractible teeth. This apparatus is commonly called
Aristotle’s lantern (Figures 22.10, 22.11A–D). It is a
real architectural marvel: a complex of hard plates
and muscles that control protraction, retraction, and
PHYLUM ECHINODERMATA
19
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Figure 22.10 The feeding complex
(Aristotle’s lantern) in sea urchins. (A) The feeding complex in a regular urchin as seen from
inside the test. (B) The feeding apparatus of
Paracentrotus (vertical section). (C) The apparatus of Cidaris (aboral view). The compasses are
removed to expose the rotules.
grasping movements of the five teeth. In many
species the entire apparatus can be rocked such that
the teeth protrude at different angles. There is great
variation in lantern structure among echinoids, but
the following brief description applies to most conditions in which it is present and well developed (e.g.,
in typical sea urchins).
The main structural elements of Aristotle’s lantern
are five vertically oriented triangular plates called pyramids (Figure 22.10). These calcareous pyramids are positioned in interambulacral spaces and are attached to
one another by comminator muscles, which provide a
rocking motion of the pyramids. The aboral edge of
each pyramid is a thickened bar called an epiphysis.
Each pyramid has a canal within which lies a tooth. The
sharp end of the tooth extends out from the oral end of
the pyramid into the mouth region. A soft dental sac of
coelomic origin covers the unhardened aboral end of
each tooth where it emerges from the top of the pyramid. As the teeth are worn down by use, more tooth
material is produced within the dental sacs and becomes calcified as it grows through the pyramid canal.
Measurements on some species indicate that, with normal wear, the teeth grow about 1 mm each week. Lying
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CHAPTER TWENTY-TWO
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Figure 22.11 Feeding and internal anatomy of echinoids. (A) A regular sea urchin (vertical section). (B)
Internal anatomy of Arbacia. (C) Arbacia (oral view). (D)
The digestive system of the sand dollar Echinarachnius
parma (aboral view). (E) A food groove on the oral surface
of a sand dollar (cutaway view). The podia are moving
food toward the mouth. (F) Dendraster excentricus in their
feeding position, half-buried in benthic sediments. (G) An
irregular urchin in its burrow.
(B)
(A)
(C)
(D)
(G)
(E)
(F)
PHYLUM ECHINODERMATA
21
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atop the main structure of the lantern, on the oral surface, are five compasses and five rotules, one of each
along each ambulacral radius. The compasses and their
associated muscles regulate hydrostatic pressure within
the gills (see below).
The teeth are protracted by the contraction of sheetlike protractor muscles that originate around the mouth,
on the interambulacral areas of the internal skeleton,
and insert on the epiphyses, near the aboral ends of each
pyramid. Their action pushes the entire lantern orally,
and also serves to spread the teeth apart as protraction
occurs. Retractor muscles originate on thick ambulacral
plates called auricles, and they insert on the oral end of
the lantern apparatus. Additional muscles associated
with the pyramids and the rotules can produce a variety
of tooth movements.
Most urchins with well developed lanterns use their
teeth to scrape algal material from the substratum and
to tear chunks of food into “bite-sized” pieces. Many
species also feed on animal matter by similar actions.
Some “regular” urchins excavate burrows in hard substrata and then feed on the algal film that develops on
the burrow wall, or else they feed on suspended particles or drift algae that enter the chamber. Other burrowers establish a feeding position at the burrow entrance
and catch floating debris with their podia and pedicellariae. Most irregular urchins (sea biscuits and heart
urchins) lack a lantern. They burrow into soft sediments
and feed on small organic particles (Figure 22.11G).
These types of urchins usually use their podia to sort
food material from the mud or sand and pass it to the
mouth.
Most sand dollars (Clypeasteroida) are detritus and
particulate feeders. They possess a highly modified
lantern with nonprotractable teeth. Most of these animals burrow completely or partially in soft sediments
and extract food particles from among the sand grains
or from the overlying water. As they plow along, a layer
of sediment passes over the aboral surface. Large nonfood particles are moved by club-shaped spines and
passed posteriorly off the body. Some species of
Clypeaster lack these large spines and instead secrete copious amounts of mucus, which prevents particles from
reaching the body surface by falling between the shorter
spines (Telford et al. 1987).
Actual food collection in most sand dollars is accomplished by podia on the oral surface. These podia are
often coated with mucus, to which small food particles
adhere. The particles are passed to the food grooves,
and podia therein move them to the mouth for ingestion
(Figure 22.3H). Apparently, at least some sand dollars
(e.g., Mellita quinquiesperforata) feed on relatively large
particles by selectively picking them out of the sediment
with special podia.
A few species of sand dollars (e.g., Dendraster excentricus) burrow into the substratum but leave the posterior part of the body extended at an angle above the sedi-
ment (Figure 20.1K and 22.11F). Dendraster traps diatoms and other particulate food in the water with its
podia and then passes the food to the mouth as described above. Larger prey, such as tiny crustaceans, are
captured by the pedicellariae. Some young sand dollars
eat high-density sand grains (especially those containing iron oxides), which they store in the gut as ballast to
help stabilize their position on the sea bottom.
Telford et al. (1983) described a unique feeding
method by the clypeasteroid Echinocyamus pusillus.
These sand dollars nestle among pebbles, which are
brought to the mouth by podia and then rotated by the
peristomial membrane while the teeth scrape off attached diatoms and organic detritus.
The digestive system of echinoids is basically a rather
simple tube extending from the mouth to the anus. The
mouth is located in the center of the oral surface or is
shifted somewhat anteriorly in some irregular urchins.
An esophagus extends aborally, through the center of
the lantern (when present), and then joins an elongate
intestine (Figure 22.11A,B,D). In most echinoids a narrow duct, called the siphon, parallels the intestinal tract
for part of its length. Both ends of the siphon open to the
intestine, providing a shunt for excess water and helping to concentrate food material in the gut lumen. In
many species, blind ceca arise from the gut near the
junction of the esophagus and intestine. The intestine
narrows into a short rectum leading to the anus, which
is located either centrally on the aboral surface, on the
posterior margin, or posteriorly on the oral surface.
Digestive enzymes are produced by the intestinal and
cecal walls, and breakdown is largely extracellular.
Holothuroidea. Most sea cucumbers are suspension
or deposit feeders. Many of the sedentary epibenthic
or nestling forms (e.g., Eupentacta, Aslia, Selenkothuria,
Psolus, Cucumaria) extend their branched, mucus-covered tentacles (Figure 22.12D,E) into the water to trap
suspended particles, including live plankton. The tentacles are then pushed into the mouth one at a time
and the food ingested (Figure 22.12F). A fresh supply
of mucus is provided by secretory cells in the papillae
of the tentacles and apparently also by gland cells of
the foregut.
More active epibenthic types (e.g., Stichopus,
Parastichopus) crawl across the substratum and use their
tentacles to ingest sediment and organic detritus (Figure
22.12C). Several studies indicate that some holothurians
(e.g., Stichopus, Holothuria) are highly selective deposit
feeders, preferentially ingesting sediments high in organic content. Sediment extracted from the gut of
Holothuria tubulosa contains a much higher percentage
of organic material than the general surrounding sediment. This animal is so adept at selective feeding that
even its fecal pellets have a higher organic content than
the environmental sediments (Massin 1980). Many apodacean holothurians burrow through the substratum by
22
CHAPTER TWENTY-TWO
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(A)
(B)
(D)
(C)
(E)
Copydot of
SEM to come
(F)
(G)
(H)
FPO slide
here/next
batch!
Figure 22.12 Feeding and internal anatomy of
holothuroids. (A) A sea cucumber (longitudinal section).
(B) Major internal organs of Holothuria tubulosa. (C) Parastichopus, a deposit feeder, in its feeding posture. (D) A
beautiful tropical holothurian (Cucumaria) showing feeding
tentacles. (E) Nodules and mucus-secreting papillae on the
tentacles of Aslia lefevrei (SEM). (F) Feeding tentacles of the
orange sea cucumber (Cucmaria minata). (G) Psolidium, a
suspension feeding holothurian; notice that the buccal
tentacles are directed upward, into the water. (H) Left respiratory tree and associated Cuvierian tubules of Holothuria
impatiens. (I) Release of Cuvierian tubules by Holothuria.
(I)
PHYLUM ECHINODERMATA
23
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peristaltic movements and ingest the sediment as they
move.
The anterior mouth is surrounded by a whorl of buccal tentacles. The esophagus (or pharynx) leads inward
and passes through a ring of calcareous plates that support the foregut and the ring canal of the water vascular
system. The esophagus joins an elongate intestine, the
anterior end of which is often enlarged as a stomach.
The intestine extends posteriorly, loops forward, and
then posteriorly again; it may be coiled (Figure
22.12A,B). The intestine terminates in an expanded rectum leading to the posterior anus. The rectal area is attached to the body wall by a series of suspensor muscles
and often bears highly branched outgrowths that extend anteriorly in the body cavity. These structures are
the respiratory trees, into which water is pumped via
the anus for gas exchange (Figure 22.12A,B,G). Digestion and absorption probably take place along the
length of the intestine.
The digestive system of sea cucumbers is associated
with two fascinating phenomena: (1) evisceration and
(2) the discharge of structures called Cuvierian tubules
(Figure 22.12H,I). Evisceration is the expulsion by muscular action of part or all of the digestive tract and sometimes other organs, including the respiratory trees and
gonads. In some forms (e.g., Holothuria) all of these
structures are expelled following rupture of the hindgut
region. In others (e.g., Thyone and Eupentacta) rupture
occurs anteriorly and the tentacular crown and foregut
are lost. Evisceration can be induced in many species by
a variety of experimental conditions (e.g., chemical
stress, physical manipulation, and crowding), but it also
occurs in nature in some species. The significance of this
process is unclear. It is viewed by some zoologists as a
Figure 22.13 Hemal system. (A) The central portion of
the hemal system and some associated structures in an
asteroid. (B) The complex hemal system of Isostichopus
badionotus (Holothuroidea), showing its association with
the gut and respiratory tree.
(A)
seasonal event associated with adverse conditions and
by others as a defense mechanism wherein the eviscerated parts serve as a decoy. In any case, the lost parts are
usually regenerated.
Cuvierian tubules are defensive structures. These organs are clusters of sticky, blind tubules arising from the
base of the respiratory tree in certain genera (e.g.,
Actinopyga and Holothuria) (Figure 22.12A,G,H). When
threatened, these cucumbers aim the anus at the potential predator, contract the body wall, and discharge the
tubules by rupturing the hindgut. The tubules are shot
onto the predator, entangling it in the sticky mass. The
Cuvierian tubules are regenerated along with any other
tissue lost during discharge.
Such elaborate defense mechanisms are not without
adaptive significance in sea cucumbers. They are common prey to a great variety of other animals, including
various sea stars, fishes, gastropods, crustaceans, and
even humans (see review by Francour 1997).
Circulation and Gas Exchange
Circulation. Internal transport in echinoderms is accomplished largely by the main perivisceral coeloms,
augmented to various degrees by the water vascular
system and the hemal system (Figure 22.13), both of
which are derived from the coelom. Fluids are moved
through these systems largely by ciliary action and in
some cases by muscular pumping. In at least one
species of sea urchin (Lytechinus variegatus) coelomic
fluid is also driven by movements of Aristotle’s lantern (Hanson and Gust 1986).
The hemal system is a complex array of canals and
spaces, mostly enclosed within coelomic channels called
perihemal sinuses. The system is best developed in
(B)
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CHAPTER TWENTY-TWO
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holothurians, in which it is bilaterally arranged, and in
crinoids, in which some of the channels form netlike
plexi. In other groups the system is radially arranged
and generally parallels the elements of the water vascular system. In these cases the hemal system consists of an
oral and an aboral hemal ring, each with radial extensions. The two rings are connected to each other by an
axial sinus lying against the stone canal (Figure 22.13A).
Within the axial sinus is a core of spongy tissue called the
axial gland, which is apparently responsible for producing some coelomocytes. As mentioned earlier, the axial
sinus often opens through pores to the stone canal and is
a source of fluid for the water vascular system.
Radial hemal channels from the aboral ring extend to
the gonads. Other radial channels arise from the oral
hemal ring and are associated with the rows of tube feet;
these channels are housed within a perihemal space
called the hyponeural sinus (Figure 22.5B). A third
hemal ring, the gastric ring, occurs in many echinoderms, including most asteroids, and is associated with
the digestive system.
Fluid is moved through the hemal system by cilia. In
asteroids and most echinoids the axial sinus bears a dorsal sac near its junction with the aboral hemal ring. The
dorsal sac pulsates, apparently aiding the movement of
fluid within the hemal channels and spaces. The hemal
system of holothurians comprises an elaborate set of
vessels (Figure 22.13B). It is intimately associated with
the digestive tract and, when present, the respiratory
trees. In many holothurians the hemal system includes
many “hearts” or circulatory pumps.
The function of the hemal system is not fully understood, but it probably helps distribute nutrients absorbed from the digestive tract. Experiments on the sea
star Echinaster graminicolus fed 14C-labeled food show
that absorbed nutrients appear in the hemal system
within a few hours after feeding and eventually concentrate in the gonads and podia (Ferguson 1984). In sea cucumbers the hemal system probably also plays a role in
gas exchange because some of the vessels are in contact
with the respiratory trees.
Gas exchange. Most echinoderms rely on thin-walled
external processes as gas exchange surfaces. Only
ophiuroids and holothurians have special internal
organs for this purpose. Given the relatively large
body sizes and volumes of many echinoderms, the
fluid transport mechanisms discussed above are of
major importance in moving dissolved gases between
internal tissues and the body surface.
Crinoids apparently exchange oxygen and carbon
dioxide across all exposed thin parts of the body wall,
especially the podia.
Gas exchange in asteroids occurs across the podia
and special outpocketings of the body wall called papulae or dermal gills (Figure 22.14A,B). These structures
are evaginations of the epidermis and peritoneum. Both
tissues are ciliated, and their cilia produce currents in
both the coelomic fluid and the overlying water. The
two currents move in opposite directions, thus creating
a countercurrent and maintaining maximum exchange
gradients across the surfaces of the papulae.
Ophiuroids possess ten invaginations of the body
wall called bursae, which open to the outside through
ciliated slits (Figure 22.9A,C). Water is circulated
through the bursae by the cilia and, in some species, by
muscular pumping of the internal bursal sacs. Gases are
exchanged between the flowing water and the body fluids. Hemoglobin occurs in the coelomocytes of a few
species of ophiuroids.
Typical sea urchins possess five pairs of “gills” that
are located in the peristomium (Figure 22.11B,C) and
have long been viewed as the major gas exchange organs. However, various authors provide evidence of a
different function (see Shick 1983). The pressure within
these “gills” changes by manipulation of the compasses
of Aristotle’s lantern. They probably function largely to
accommodate pressure changes in the peripharyngeal
coelom during feeding movements of the lantern complex, and perhaps to provide an immediate oxygen supply to the associated muscles. The main gas exchange
structures in these urchins are apparently thin-walled
podia that operate on a countercurrent system similar to
that associated with the papulae of asteroids (Figure
22.14C,D).
Irregular sea urchins and sand dollars bear highly
modified podia on the aboral petaloids (the five ambulacral regions of the fused skeleton, or test) (Figure
22.3G). The external parts of these podia are flaplike and
thin-walled and serve as the main gas exchange surfaces. A countercurrent flow occurs between the water
vascular system fluid in the podia and the sea water,
and between the water vascular system fluid in the ampullae and the coelomic fluid (Figure 22.14E). Fenner
(1973) provides a thorough examination of the respiratory function of echinoid podia.
We have already described the respiratory trees of
certain holothurians. Water is pumped in and out of the
hindgut and branches of the respiratory trees, and gases
are exchanged between the water and the coelom and
hemal system. This device is augmented by exchange
across the podia, which is facilitated by a countercurrent
system. Hemoglobin occurs in the coelomocytes of
many holothurians.
Excretion and Osmoregulation
Excretion. In most echinoderms dissolved nitrogenous wastes (ammonia) diffuse across body surfaces
to the outside. This type of excretion occurs across the
podia and papulae in asteroids and is suspected to
occur across the respiratory trees in holothurians. At
least some excretion by simple diffusion probably
takes place in most echinoderms. Precipitated nitrogenous material and other particulate wastes are
PHYLUM ECHINODERMATA
25
UNCORRECTED PAGE PROOFS
Figure 22.14 Gas exchange in echinoderms. (A)
A portion of the aboral surface of Asterias. Note the
digitiform papulae and their surrounding structures.
(B) An asteroid papula (section). This structure is
lined by the peritoneum and is filled with coelomic
fluid. (C) An ampulla and podium (longitudinal section) of Strongylocentrotus purpuratus (Echinoidea).
The arrows represent the countercurrents between
the ambient sea water, the fluid of the water vascular system, and the coelomic fluid. (D) Three lamelliform ampullae from Strongylocentrotus. Gases are
exchanged between the fluids of the water vascular
system and the coelom. (E) A “respiratory” podium
and ampulla (section) of the irregular sea urchin
Echinocardium. The arrows represent the countercurrents.
phagocytosed by certain coelomocytes in the body
fluids and then discharged by various methods.
In asteroids, waste-laden coelomocytes accumulate
in the papulae, which then pinch off their distal ends,
expelling the cells and waste material. Some studies indicate that the rectal glands may also be involved in excretion. In ophiuroids it is suspected that coelomocytes
deliver wastes to the bursae, where they are released.
Phagocytic coelomocytes in echinoids accumulate
wastes and transport them to the podia and gills for release. In holothurians particulate wastes are carried by
coelomocytes to the respiratory trees, gut, and even the
gonads, and released to the outside through the plumbing systems of these organs. Crinoid coelomocytes deposit wastes in tiny pockets along the sides of the ambulacral grooves, but discharge has not been observed.
Osmoregulation. Echinoderms are generally considered to be strictly marine, stenohaline creatures. Consequently, they do not have problems of osmotic and
ionic regulation. However, a number of species have
been reported from brackish water. For example, Asterias rubens (Asteroidea) has been collected from the
Baltic Sea (8‰), Ophiophragmus filagraneous (Ophiuroidea) from Cedar Key, Florida (7.7‰), and various
holothurians from the Black Sea (18‰) (Binyon 1966).
Obviously some mechanism allows them to survive
in these low salinities.
The evidence to date suggests that echinoderms are
osmoconformers. Both water and ions pass relatively
freely across thin body surfaces, and the tonicity of the
body fluids varies with environmental fluctuations.
There appears to be some ionic regulation through active transport, but it is minimal.
Nervous System and Sense Organs
The secondarily derived radial bauplan of echinoderms
is clearly reflected in the anatomy of their nervous systems and the distribution of their sense organs. The nervous system is decentralized, somewhat diffuse, and
without a cerebral ganglion. There are three main neuronal networks, integrated with one another and developed to various degrees among the classes. These networks are the ectoneural (oral) system, the hyponeural
(deep oral) system, and the entoneural (aboral) system.
The ectoneural system is predominately sensory, although motor fibers do occur; the hyponeural system is
largely motor in function. The entoneural system is absent from holothurians and reduced to different degrees
in other groups—except the crinoids, in which it is the
26
CHAPTER TWENTY-TWO
UNCORRECTED PAGE PROOFS
primary nerve component and serves both motor and
sensory functions.
The three nervous “systems” are interconnected by a
nerve net derived primarily from the ectoneural and entoneural components. The nerve net is often described
as a subepidermal plexus, but it gives rise to intraepidermal neurons and clearly has an intimate association
with the epithelium.
Except for the crinoids, in which the entoneural component dominates, the most obvious nerves in echinoderms are derived from the ectoneural system. A circular or pentagonal circumoral nerve ring lies just
beneath the oral epithelium and encircles the esophagus. From this ring arise radial nerves that extend along
each ambulacrum. In sea stars, for example, these radial
nerves appear as a distinct V-shaped thickening in the
epidermis of each ambulacral groove (Figure 22.5B). In
some cases the entoneural components of the nerve
plexus are also produced as radial cords, such as those
along the lateral margins of the arms of asteroids. The
hyponeural system generally parallels the nerves of the
ectoneural system. Hyponeural neurons are subepidermal and lie near the hyponeural sinus of each ambulacral area (Figure 22.5B). These neurons give rise to
motor fibers and ganglia in the tube feet.
Sensory receptors are largely restricted to relatively
simple epithelial structures innervated by a plexus of
the ectoneural system. Sensory neurons in the epidermis respond to touch, dissolved chemicals, water
currents, and light. They are frequently associated with
outgrowths of the body wall, such as spines and
pedicellariae. Special photoreceptors occur in asteroids
as optic cushions, each of which comprises a cluster of
pigment-cup ocelli at the tip of an arm. Statocysts are
known in some holothurians, and georeception is presumed to be the function of structures called sphaeridia
in certain echinoids. Chemoreception has not been well
studied in echinoderms, but there is some evidence that
the buccal tentacles of holothurians and the oral podia
of some echinoids are sensitive to dissolved chemicals.
Chemoreception in asteroids appears to depend largely
on direct contact, although distance chemoreception is
reported in some species.
In spite of their rather simple nervous system and
their lack of specialized sense organs, many echinoderms engage in complex behaviors. As is so often the
case in such matters, there is still much to be learned
about the functional mediation between the circuitry of
the nervous system and the observed behavioral responses, as in the coordination of the podia during locomotion. Most echinoderms also exhibit distinct righting
behaviors when overturned. These actions probably involve touch, georeception, and perhaps photoreception.
Orientation to currents is known in some sand dollars
and in many ophiuroids and crinoids. There is even evidence to support the contention that some degree of
learning occurs in echinoderms (see Valentincic 1983).
Reproduction and Development
Regeneration and asexual reproduction. Most echinoderms are capable of regenerating lost parts. Even the
casual observer of tidepool life will encounter a sea
star regenerating a new arm, or notice the suckers of
the podia left on a rock from which a sea star or urchin
has been pulled free. Lost suckers are quickly
replaced by regeneration. We have already described
the dramatic processes of evisceration and expulsion
of Cuvierian tubules—in both cases, the lost organs
are replaced. Studies on regeneration in asteroids
have put to rest the tales of oystermen who once
claimed that chopping sea stars into small pieces
resulted in the regeneration of an entire new animal
from each part. While it is true that a damaged animal
can grow new arms if a substantial portion of the central disc remains intact, an isolated arm soon dies. The
exception to this generality is Linckia, which can
regenerate an entire individual from a single arm, the
regenerating stage being appropriately called a comet
(Figure 22.15). Ophiuroids and crinoids frequently
cast off arms or arm fragments when disturbed, and
then regenerate the lost part. Such autotomy (voluntarily casting off an appendage) is also documented
for certain asteroids. The Pacific coast ochre star,
Pisaster ochraceus, autotomizes arms at their junction
with the central disc when confronted by predators
(e.g., the sea star Pycnopodia).
Asexual reproduction occurs in some asteroids and
ophiuroids by a process called fissiparity, wherein the
central disc divides in two and each half forms a complete animal by regeneration. When the small six-rayed
brittle star Ophiactis divides, each half retains three
arms. Asexual fission also occurs in some holothurians,
but the process is not well understood.
Sexual reproduction. The majority of echinoderms
are dioecious, but hermaphroditic species are known
among the asteroids, holothurians, and especially the
ophiuroids. The reproductive system is relatively simple and is intimately associated with derivatives of
the coelom. The gonads are usually housed within
peritoneally lined genital sinuses. Holothurians are
unique among echinoderms in possessing a single
gonad, which lies dorsally in the CD interambulacrum (Figure 22.12B). A single gonoduct opens
between the bases of two dorsal buccal tentacles or
just posterior to the tentacular whorl.
Crinoids lack distinct gonads. The gametes arise
from the peritoneum of special coelomic extensions
called genital canals in the pinnules on the proximal
portion of each arm. There are no gonoducts; gametes
are released by rupture of the pinnule walls. Ophiuroids
possess from one to many gonads attached to the peritoneal side of each bursa adjacent to the bursal slits
(Figure 22.9A). Gametes are released into the bursae and
expelled through the slits.
PHYLUM ECHINODERMATA
27
UNCORRECTED PAGE PROOFS
Figure 22.15 Regeneration in Linckia. (A) Initial regeneration from a single arm, here yielding a central disc with
dual madreporites and five new rays. (B) At a later stage,
the animal has a single madreporite and the normal ray
number.
Asteroids and echinoids possess multiple gonads
with gonoducts leading to interambulacral gonopores
(Figures 22.8C, 22.11B). Sea daisies have a pair of gonads
in each ambulacrum (Figure 22.3E). “Regular” sea
urchins contain five gonads, one lying along the inside
of each interambulacral radius. The gonopores are located on the five interambulacral genital plates surrounding the periproct (Figures 22.5F, 22.11A). The periproct
and anus have migrated posteriorly in irregular urchins
and sand dollars, but the genital plates remain more or
less centrally located on the aboral surface. In many of
these animals there are only four (and sometimes fewer)
gonads, one being lost along the line of migration of the
anus. In such cases there is a corresponding reduction in
the number of gonopores. In all urchins one of the genital plates is perforated and doubles as the madreporite.
Life history strategies among echinoderms vary from
free spawning followed by external fertilization and indirect development to various forms of brooding and
direct development. Spawning has been observed in nature in only a few species of echinoderms. Some studies
indicate that spawning is mostly a nocturnal event,
wherein the animals assume characteristic postures
with their bodies elevated off the substratum.
Gametogenesis in at least some asteroids and echinoids
is regulated by photoperiod (Pearse et al. 1986), which
in turn ensures more or less synchronous spawning
among members of the same population. In some
species of free-spawning asteroids the females release
pheromones that attract the sperm from nearby conspecific males (Miller 1989).
Brooding is especially common among boreal and
polar species in all groups of echinoderms and in certain
deep-sea asteroids, whose environments are unfavorable for larval life. As expected, brooding species pro-
duce fewer but larger and yolkier eggs than do their
free-spawning counterparts.
Brooding methods vary. Among the crinoids,
Antedon and a few others cement their eggs to the epidermis of the pinnules from which they emerge (Figure
22.16A,B). Once the eggs are fertilized by free sperm, the
embryos are held by the parent until hatching. Most
brooding asteroids hold their embryos on the body surface. One species (Asterina gibbosa) cements its eggs to
the substratum, and another (Leptasterias tenera) broods
its early embryos in the pyloric stomach before moving
them to the outer body surface (Hendler and Franz
1982). Sea daisies brood within the gonads and apparently release juveniles that may drift for some time before settling. Brooding is common among ophiuroids.
Sperm enter the bursae and fertilize the eggs, and the
embryos are held within these sacs during development. Some echinoids brood their embryos among clusters of spines on the body or, in the case of sand dollars,
on the petaloids. Brooding holothurians usually carry
their embryos externally (Figure 22.16C), but some
species of Thyone and Leptosynapta brood inside the
coelom.
Development. The tremendous numbers of eggs
produced by many echinoderms and the ease with
which they can be reared in the laboratory have made
these animals favorite objects of study by embryologists. Much of our information about the biology of
animal fertilization and early development comes
from over a century of work focusing particularly on
urchins and sea stars. In addition, the early ontogeny
of some echinoderms has served as a model of
deuterostome development against which many
other developmental patterns are measured. Except in
brooding species, in which development is modified
by large amounts of yolk, the sequence of ontogenetic events is remarkably similar throughout the phylum. We cannot cover the vast amount of information
on this subject, and present only a brief overview of
indirect development, emphasizing urchins and asteroids and including some comparative comments on
other taxa. (For a review and detailed bibliography,
see Wray 1997.)
The ova of free-spawning echinoderms are usually
isolecithal with relatively small amounts of yolk.
Cleavage is radial, holoblastic, and initially equal or
subequal and leads to a spacious coeloblastula. In some
groups, such as urchins, cleavage preceding the blastula
becomes unequal, resulting in blastomere tiers of vegetal mesomeres underlain by slightly larger macromeres
and a cluster of micromeres at the animal pole. (These
terms refer here only to the relative sizes of the cells, and
are not to be confused with the same terms as they are
used in describing spiral cleavage.) The coeloblastula
usually becomes ciliated and breaks free of the fertilization membrane as a swimming embryo.
28
CHAPTER TWENTY-TWO
UNCORRECTED PAGE PROOFS
Figure 22.16 Brooding in a crinoid and a holothurian.
(A) Portion of an arm of Antedon. The ova are housed
within a pinnule (lower portion) and released to exterior
(upper portion). (B) Part of a pinnule of the crinoid
Phixometra with developing young. (C) Cucumaria crocea,
brooding its young.
The blastula flattens slightly at the animal pole, forming the gastral plate, from which some cells proliferate
into the blastocoel as primary or larval mesenchyme. In
most echinoids these cells are the micromeres. The surrounding macromeres are the presumptive entoderm
and adult mesoderm, and the vegetal mesomeres are
the presumptive ectoderm. A coelogastrula is produced
by invagination of the animal pole cells. The blastopore
typically forms the anus; the archenteron grows to connect with a stomodeal inpocketing that forms the
mouth. Before the gut is complete, however, the inner
end of the archenteron proliferates secondary mesenchyme, as well as one or two evaginations of mesoderm, into the blastocoel. Thus, coelom formation (enterocoely) is by archenteric pouching.
During the later stages of gastrulation and coelom
development, the embryo assumes bilateral symmetry
and eventually becomes a swimming larva (Figure
22.17). Planktotrophic echinoderm larvae use bands of
cilia to swim and to create feeding currents. Lecithotrophism, however, is common and has apparently
evolved several times within some echinoderm classes.
The primary mesenchyme contributes to the formation of larval muscles and in some cases calcareous
spicules or ossicles (Figure 22.17G). The adaptive significance of these larval skeletons has been explored by
several authors. Suggestions about the functions of the
larval skeleton include defense, physical support, and
sites of muscle attachment. These ideas are summarized
by Pennington and Strathmann (1990), who also provide evidence that the skeletal elements enhance passive orientation of the larva in the water. Most feeding
echinoderm larvae are oriented with the anterior end directed upward. The bulk of the skeleton lies in the posterior part of the larva, creating a higher density at the
rear end. The larvae of some echinoderms contain distinctive reddish pigment spots that may be involved in
photochemical energy-producing reactions (Ryberg
1980).
In order to understand the development of echinoderm larvae and their eventual, remarkable metamorphosis to radially symmetrical adults, it is necessary to
examine carefully the embryogeny and fates of the
coelomic spaces. Although there are some differences in
the details of these events among groups, they are similar enough to generalize for our purposes. The initial
archenteric pouching typically occurs from the blind
end of the developing gut, either as a pair of coeloms or
as one cavity that divides into two. These coeloms pinch
off another pair of cavities posteriorly, and then a third
pair between the anterior and posterior ones (Figure
22.18A). From front to back, these pairs of coelomic
spaces are called right and left axocoels, hydrocoels,
and somatocoels. These spaces correspond to the protocoels, mesocoels, and metacoels of other trimeric
deuterostomes. As illustrated, the left axocoel and left
hydrocoel do not fully separate, but remain connected
to one another by the stone canal. From the left axocoelhydrocoel complex arises a hydrotube, which grows to
the dorsal surface of the larva and opens to the outside
via a hydropore. In most cases the right axocoel disappears and the right hydrocoel becomes associated with
the hydrotube as the dorsal sac.
As we explain metamorphosis, it will help you to
note the fates of these various coelomic derivatives,
PHYLUM ECHINODERMATA
29
UNCORRECTED PAGE PROOFS
(A)
Sensory tuft
(B)
(G)
(H)
Adhesive pit
Mouth
Ciliary
bands
Anus
(C)
Adhesive pit
(D)
Mouth
Figure 22.17 Echinoderm larval types. (A) Vitellaria
larva of a crinoid. (B,C) Bipinnaria and later brachiolaria
larvae of a sea star. (D) Ophiopluteus larva of a brittle star.
(E) Echinopluteus larva of a sea urchin. (F) Auricularia larva
of a sea cucumber. (G) Isolated larval spicule from the
sand dollar Dendraster. (H) Late pentacula stage (postlarva)
of a sea cucumber. Skeletal ossicles and juvenile podia are
present.
Anus
(E)
(F)
Mouth
Anus
TABLE 22.1
which are outlined in Table 22.1. As the time for metamorphosis approaches, the larva swims to the bottom
and selects and attaches to an appropriate substratum.
In general, the larval left and right sides become the
adult oral and aboral surfaces, respectively, although
this pattern often varies from a precise 90° reorientation.
The remarkable change from bilateral to radial symmetry involves shifts in the positions of the mouth and
anus. In many cases the embryonic openings disappear,
and the stumps of the foregut and hindgut migrate beneath the body surface to their adult positions. The
foregut swings from its larval anteroventral location to
the left side, and the hindgut moves anteriorly and to
the right (Figure 22.18B,C). As the foregut migrates, it
presses into the wall of the left hydrocoel, which encircles the foregut as the precursor of the ring canal. Once
Adult fates of the major coelomic derivatives
in generalized enchinoderm development
Embryonic coelomic
structure
Adult fate
Right somatocoel
Aboral perivisceral coelom
Left somatocoel
Oral perivisceral coelom; genital sinuses;
most of the hyponeural sinus
Right axocoel
Largely lost
Hydropore
Incorporated into madreporite
Hydrotubule and dorsal sac
Parts of madreporic vesicle and ampulla
Stone canal
Stone canal
Left hydrocoel
Ring canal; radial canals; lining of tube feet
lumina, plus other components of the water
vascular system, including Tiedemann’s
bodies and polian vesicles
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CHAPTER TWENTY-TWO
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Figure 22.18 (A–D) The development of the coelom and its derivatives in an asteroid. (E) Metamorphosis in the same animal.
affixed in the adult positions, the mouth and anus reopen. The ring canal grows radial extensions (Figure
22.18D,E) destined to become the radial canals and podial linings, and outgrowths of the left somatocoel produce the hyponeural sinus. Aborally, the madreporite
complex arises from various parts of the left axocoel and
its derivatives plus the dorsal sac, and marks the position of the CD interambulacrum. The axial sinus arises
from an outpocketing of the left axocoel.
As these transformations take place, most of the larval structures are lost and the juvenile assumes benthic
life. Many echinoderm larvae appear to settle preferentially near conspecific adults. In at least some species
(e.g., Dendraster excentricus) successful metamorphosis is
triggered by pheromones that are released by adults
and act on the larval nervous system.
Echinoderm Phylogeny
In spite of a rich fossil record and many decades of
work, the origin and subsequent evolution of echinoderms remain highly controversial issues. There have
been a number of popular and competing ideas on these
matters, as evidenced by the chronic instability of echinoderm classification. We focus here on phylogeny at the
class level, and assume that each class is in fact a monophyletic group. Our treatment relies largely on adult
morphology. Larval types have been used by some
workers, but details of larval form do not always correlate well with adult traits (see Wray and Bely 1994 for a
discussion of the evolutionary forces influencing echinoderm larvae).
The echinoderm lineage probably originated with
the Precambrian invasion of epibenthic habitats by an
ancestral burrowing deuterostome. The line diversified
rapidly, and most of the fundamental body plans within
the phylum were probably established in the early
Cambrian. Echinoderm diversity reached its zenith during the early to mid-Paleozoic, but by the beginning of
the Mesozoic it had declined greatly in terms of higher
taxa, leaving only five major groups persisting to recent
times. Some fossil forms are shown in Figure 22.19.
The origin of echinoderms probably involved the
evolution of the endoskeleton composed of plates with
the unique stereom structure. Evidence suggests that
the skeleton originated prior to the adoption of radial
symmetry, the latter marking the appearance of the first
PHYLUM ECHINODERMATA
UNCORRECTED PAGE PROOFS
(A)
(B)
(C)
(E)
(D)
(F)
(G)
(I)
(H)
Figure 22.19 Fossil echinoderms and near-echinoderms. (A) Dendrocystites, a carpoid. Carpoids were early
Cambrian animals that probably shared a common ancestor with the true echinoderms. Note the absence of radial
symmetry. (B) The helicoplacoid Helicoplacus, a triradiate
echinoderm from the Lower Cambrian. This creature, with
spiral ambulacra, may represent the ancestral echinoderm
bauplan. (C) Camptostroma roddyi, an early Cambrian
edrioasteroid with five ambulacra arranged in a manner
suggesting derivation from a triradiate form. (D)
Steganoblastus, a stalked edrioasteroid showing clear pentamery. (E) A generalized cystoid. (F) Lepadocystis, a
stalked Ordovician cystoid. Note also the attached edrioasteroid. (G) Eifelocrinus, an extinct crinoid. (H) The
eocrinoid Macrocystella. (I) A generalized blastoid from the
Carboniferous. (J) Volchovia, a strange extinct echinoid.
(J)
31
32
CHAPTER TWENTY-TWO
UNCORRECTED PAGE PROOFS
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(A)
Figure 22.20 (A) A cladogram depicting one
hypothesis about the origins of some important
synapomorphies among the major groups of
echinoderms. (B) A competing hypothesis (extant
25
taxa only) illustrating a slightly different view on
22
19
24
the placement of the ophiuroids. The origin of the
23
18
17
echinoderm lineage involved an escape from infau21
16
14
nal life with the evolution of a supportive system of
20
endoskeletal plates with a stereom structure (1)
and the use of external ciliary grooves for suspen15
sion feeding (2). This proto-echinoderm condition
is represented in the fossil record by the carpoids.
13
The first true echinoderms may have been the heli12
9
11
coplacoids, whose appearance was marked by the
8
10
origin of triradial symmetry with three spirally
arranged, open ambulacral grooves (3) and a
•7 (a)
water vascular system (4), probably with the
6
5
madreporite opening near the mouth. The immeHypothesis A
diate common ancestor of the modern lineages (a)
4
may have been similar to the extinct
3
Camptostroma, with pentaradial symmetry (5) evi2
denced by five open ambulacral grooves, mouth
1
and anus on the oral surface (6), and attachment
Trimeric, burrowing
coelomate ancestor
to the substratum by the aboral surface (7). From
this ancestral form, the Crinoidea and Cystoidea
diverged with the evolution of arms or brachioles
(B)
bearing open ciliated grooves used for suspension
feeding (8) and the loss of the external
madreporite (9). The origin of the sister clade
(Asteroidea, Ophiuroidea, Echinoidea, and
25
Holothuroidea) involved the movement of the
24
22
anus to the aboral surface (10), and a change asso19
23
ciated with the orientation of the body with the
18
14
17
oral surface against the substratum (11). In this
16
“new” position, these echinoderms adopted alternative feeding modes and a somewhat errant
21
lifestyle; the podia became suckered (12) and used
20
for locomotion rather than feeding. The
madreporite migrated along the CD interambulacrum to the aboral surface (13). The asteroids
26
9
arose with the evolution of five arms broadly conHypothesis B
8
nected to a central disc (14). The remaining three
groups have closed ambulacral grooves (15) in
13
common. The ophiuroids invaded soft substrata
12
11
and lost the podial suckers (16). In addition, they
10
evolved five highly articulated rays, with internal
vertebral plates in each arm “segment” (17), and
secondarily lost the anus (18). The madreporite
migrated back to the oral surface along the CD
interambulacrum (19). The echinoid–holothurian
clade arose with the extension of the ambulacral
plan. Both cladograms include some unresolved problems.
grooves along the sides of the body from the oral to the
Both treat the evolution of ambulacral rays (arms) as conaboral pole (20), thereby reducing the aboral surface to a
vergent, arising once in the crinoids and again in the
small region around the anus (21). The echinoids evolved
asteroids and ophiuroids; cladogram A indicates that this
with the fusion of the skeletal plates, which formed a rigid
condition is also convergent in the asteroids and ophiglobular or discoidal test (22). The origin of the holothuriuroids. Cladogram A treats the oral madreporite position
ans involved a reduction of the skeletal plates to isolated
in ophiuroids as secondary, and thus convergent with the
ossicles (23), movement of the madreporite internally (24),
same condition in groups that diverged earlier (e.g.,
and elongation of the fleshy body on the oral–aboral axis
crinoids). In addition, cladogram B accepts as convergent
(25). Cladogram B unites the asteroids and ophiuroids into
the closed ambulacral grooves of ophiuroids and those of
a single clade on the basis of the five-rayed (26) body
echinoids and holothurians.
PHYLUM ECHINODERMATA
Figure 22.21 Two orthodox evolutionary trees of the
echinoderms. (A) A tree compatible with cladogram A in
Figure 22.20. (B) A “traditional” tree of the extant classes,
compatible with cladogram B in Figure 22.20.
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O
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(B)
C
“true” echinoderms as shown in the cladograms in
Figure 22.20. The first dichotomy separates the main
echinoderm clade from a now extinct group called the
carpoids (Figure 22.19A). Many authors view the carpoids as echinoderms and place them in a separate subphylum, the Homalozoa. Although they possessed
stereom ossicles, they were not radially symmetrical,
and the nature of their water vascular system (if they
had one) is uncertain. Because they lacked some of the
fundamental defining characteristics of the echinoderms, they are best considered as an early pre-echinoderm group. These early epibenthic creatures were
probably suspension feeders, and they bore a grooved
arm or brachiole that apparently led to the mouth.
The first true echinoderms may have been the helicoplacoids (Figure 22.19B). These odd creatures appeared in the early Cambrian and died out soon thereafter. They were spindle-shaped, with spirally arranged
skeletal plates and three ambulacra. The mouth was located on one side of the body rather than apically, so
these animals were not constructed on an obvious
oral–aboral axis as are modern echinoderms. It appears
that the skeletal plates articulated somewhat, and the
helicoplacoids may have been either surface plowers or
attached with the oral side against the substratum. The
ambulacra probably conveyed detrital or suspended
food material to the mouth. Some authors speculate that
the “lateral” placement of the mouth in helicoplacoids is
indicative of the origin of the metamorphic events during the conversion from bilateral to radial symmetry
seen in extant echinoderms. This conversion must have
involved some major changes in genes that influence
fundamental body architecture. Since echinoderm larvae retain their ancestral bilateral condition, the expression of genes that dictate radiality must occur relatively
late in development. According to Lowe and Wray
(1997), this change involved major modifications of the
roles of Hox genes during the early evolution of the
echinoderm bauplan.
The next major dichotomy separates the crinoids and
extinct cystoids (Figure 22.19E) from other echinoderms
(Figures 22.20, 22.21). Paul and Smith (1984) suggest that
the extinct genus Camptostroma (Figure 22.19C) is similar
to what may have been the common ancestor of these
two monophyletic sister clades. It is the earliest known
pentaradial echinoderm, with the five ambulacra developed in a 2-1-2 pattern, perhaps derived from the triradial pattern of the helicoplacoids. Camptostroma is usually
a
UNCORRECTED PAGE PROOFS
Ancestor
33
34
CHAPTER TWENTY-TWO
UNCORRECTED PAGE PROOFS
assigned to a wholly extinct group called the Edrioasteroidea (Figure 22.19D). The lineage that persists
today as the class Crinoidea includes extinct forms that
bore attachment stalks arising from the aboral surface
(e.g., extinct crinoids and cystoids, Figure 22.19E–H).
These animals suspension fed by orienting with the oral
side upward and using the open ambulacral grooves in
the arms or brachioles for transporting food to the
mouth.
The origin of the lineage that includes the asteroids,
ophiuroids, echinoids, and holothurians involved the
adoption of other feeding modes and used the water
vascular system largely for locomotion. These animals
became more or less errant and, with the exception of
the holothurians, oriented themselves with the oral side
against the substratum. The phylogeny of these classes
is controversial and as yet unsettled. On the basis of its
temporal appearance in the fossil record, Paul and
Smith (1984) place the extinct genus Stromatocystites
(Figure 22.21A) at the base of this line and indicate that
the familiar benthic-feeding asteroids, ophiuroids, and
echinoids did not appear until later (Ordovician), and
that the holothurians appeared even more recently
(early Mississippian). However, the emphasis on fossil
chronology must be viewed with some caution, even in
a group as well represented as the echinoderms. The evidence preserved in rocks is still fragmentary and may
easily be misleading. The holothurians, for example,
leave only isolated ossicles from which to draw inferences. Deep-sea ophiuroids and asteroids probably did
not fossilize well, and the skeletons of others are known
to disarticulate soon after death. Although we cannot ignore the fossils, we cannot base an entire evolutionary
scenario on what they seem to tell us. Based on fossil evidence, sea urchins (Euchinoidea) are more diverse
today than anytime in the past.
The close relationship between the echinoids and
holothurians is accepted by most specialists, but there is
much debate about whether the ophiuroids are closer to
the asteroids or to the echinoid–holothurian clade. Two
alternative cladograms are presented and discussed in
Figure 22.20.
In summary, the evolution of a mesodermal skeleton
of stereom ossicles was followed by the appearance of a
water vascular system and pentaradial symmetry. These
features allowed escape from infaunal life to epibenthic
surface dwelling. The water vascular system probably
originally served for suspension and perhaps detritus
feeding, facilitated first by simple ciliary tracts, as in the
carpoid brachioles, and later by the development of ambulacral grooves in the helicoplacoids, which persisted
in the crinoids. Radial symmetry became the popular
architecture among the echinoderms and enhanced
their new lifestyle. The use of suckered podia for locomotion was a secondary event that provided a means of
exploiting new habitats and food resources. Later, in
ophiuroids, echinoids, and holothurians, the ambulacral
grooves closed, with a concomitant loss of feeding functions, and feeding became the responsibility of other
structures (e.g., podia, buccal tentacles, teeth).
There is no doubting the success of the basic echinoderm bauplan. The combined qualities of the supportive endoskeleton, coelomic water vascular system, and
pentaradial symmetry are unique to these animals and
have provided the basis for their diversification along
several distinct lineages.
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