Chapter 16
The Echinoderms
Evolutionary Perspective
• Flourished in 400 million year-old seas
– Many were attached suspension feeders.
– 12 of 18 classes now extinct
– Remain a major component of marine
• Characteristics
Calcareous endoskeleton (ossicles)
Adults with pentaradial symmetry
Water-vascular system
Complete digestive tract
Hemal system
Nervous system consisting of nerve net,
nerve ring, and radial nerves
Evolutionary Perspective
• Deuterostomes
– Closest relatives
• Hemichordata
• Chordata
– Evolved from bilaterally symmetrical
• Larval stages are bilaterally symmetrical.
• Some extinct forms bilaterally symmetrical.
– Evolved from radially symmetrical
• Arkarua tentatively identified as oldest extinct
• Pentaradially symmetrical
Figure 16.1 Evolutionary relationships of the
echinoderms to other animals.
Echinoderm Characteristics
• Pentaradial symmetry
– Body parts arranged in fives around
an oral-aboral axis.
– Some secondarily bilateral
– Evolution of the skeleton may
account for pentaradial form
(figure 16.2).
Figure 16.2 Pentaradial
symmetry. (a) Body parts are
arranged in fives around an
oral-aboral axis.
(b) Arrangement of the body in
fives means skeletal joints are
not directly opposite one
another. This arrangement
may make the skeleton
stronger than if joints were
opposite one another.
Echinoderm Characteristics
• Water vascular system (figure 16.3)
– Water-filled canal and tube feet
– Ring canal opens to outside via stone canal and
– Polian vesicles function in water storage.
– Tube feet
• Muscular ampulla
• Often suction cup at distal end (may also be blunt or
– Functions
Exchanges of respiratory gases and wastes
Sensory functions
• Hemal system
– Distributes nutrients and large molecules
Figure 16.3 The water-vascular system of a sea star.
Class Asteroidea
• Sea stars
– Hard or sandy substrates
– Moveable and fixed spines roughen
body surface.
– Dermal branchiae (papulae)
• Gas exchange
– Pedicellariae
• Pincerlike
• Clean and protect body surface
– Tube feet with suction disks
Class Asteroidea
• Maintenance Functions
– Predators and detritus feeders
• Ingest whole prey
• Many are bivalve predators
– Internal transport of gases, nutrients, and
metabolic wastes by diffusion and hemal
– Gas exchange and excretion by diffusion
across dermal branchiae
– Nervous system
• Nerve ring, radial nerves, nerve net
– Sensory receptors
• Widely distributed over body surface
• Photoreceptors at tips of arms (specialized tube
Figure 16.4 Body wall and internal anatomy of a
sea star.
Figure 16.5
Internal structure
of a sea star.
Regeneration, Reproduction,
and Development
• Regeneration
– Broken arm replaced
– Entire sea star from portion of central disk
• Asexual reproduction in some
– Regeneration after division of central disk
• Sexual reproduction
– Dioecious
– Two gonads per arm (figures 16.4 -16.5)
– External fertilization and planktonic larval
development (figure 16.6)
Figure 16.6
Development of a sea
Class Asteroidea
• Sea Daisies
– Previously class
– Highly modifies
• Lack arms
• 1 cm diameter
• Digestion and
absorption of
decaying organic
Figure 16.7 A sea daisy
(Xyloplax medusiformis).
Class Ophiuroidea
• Basket stars and brittle stars
• Arms long, sharply set off from central
disk (highly branched in basket stars)
• No dermal branchiae or pedicellariae
• Tube feet lack suction disks.
• Madreporite on oral surface
• Muscles and articulating ossicles
produce snake-like movements of arms.
– Water vascular system is not used in
Figure 16.8 Class
Ophiuroidea. (a) A brittle
star (Ophiopholis
aculeata). (b) A basket
star (Gorgonocephalus
Class Ophiuroidea
• Maintenance functions
– Predators and scavengers
• Arms sweep substrate.
• Basket stars are suspension feeders.
– Wave arms and trap plankton on mucuscovered tube feet
– Coelom confined to central disk.
• Distribution of nutrients, gases, wastes
– Ammonia lost by diffusion across
tube feet and bursae.
Figure 16.9 Oral view of the disk of the brittle star
Regeneration, Reproduction,
and Development
• Regeneration
– Lost arms
– Autotomy common
– Fission across central disk
• Sexual Reproduction
– Dioecious
– Gonads associated with bursa
– Gametes released into bursa
• Eggs may be retained and fertilized within bursa or
released for external fertilization.
• Development
– Within bursa or as planktonic larvae
– Ophiopluteus is planktonic and metamorphoses
to adult.
Class Echinoidea
• Sea urchins, sand dollars, heart
– Sea urchins—hard substrates
– Sand dollars and heart urchins—sand or
mud just below surface
• Sea urchin skeleton
– Test of 10 sets of closely fitting plates
• Ambulacral plates with openings for tube
• Interambulacral plates articulate spines
• Pedicellaria with long stalk (may be
Figure 16.10 (a) A sea urchin
(b) A sand dollar.
Class Echinoidea
• Water-vascular system
– Radial canal runs along inner body
– Tube feet with ampullae and suction
– Madreporite opens at aboral surface.
• Spines
– Locomotion and burrowing
Figure 16.11
(a)Internal anatomy
of a sea star.
(b) Aristotle’s lantern.
Class Echinoidea
• Maintenance Functions
– Feed on algae, bryozoans, animal
• Aristotle’s lantern (figure 16.11b)
• Complete digestive tract (figure16.11a)
– Circulation
• Coelomic fluids
– Gas exchange
• Diffusion across gill membrane
surrounding mouth (figure 16.11a)
Reproduction and
• Dioecious
• Gonads on internal body wall
• One gonopore in each of 5 genital
– Sand dollars have 4 gential ossicles
and gonopores.
• External fertilization and
planktonic larvae
– Metamorphosis to adult
Class Holothuroidea
• Sea cucumbers
• Hard and soft substrates in all
• Elongate along oral-aboral axis
• One side flattened and “ventral”
– Secondary bilateral symmetry
• Oral tube feet enlarged and modified
as tentacles.
• Body wall thick and muscular with
microscopic ossicles.
Figure 16.12 Class Holothuroidea (Parastichopus
Class Holothuroidea
• Water-vascular system
Madreporite internal
Filled with coelomic fluid
Ring canal encircles oral end.
1-10 Polian vesicles
Radial canals run between oral and
aboral poles.
– Tube feed with ampulae and suction
• 3 of 5 rows on flattened “ventral” surface
used for attachment.
Figure 16.13
Internal structure
of the sea
cucumber, Thyone.
Class Holothuroidea
• Maintenance Functions
– Feed on particulate organic matter by
sweeping substrate with mucus-covered
• Long, looped intestine
– Circulation
• Coelomic fluid
– Gas exchange
• Respiratory trees attach to rectum.
– Defense
• Toxins in body wall
• Evert Cuverian tubules
Class Holothuroidea
• Reproduction and Development
– Dioecious
– Single gonad and single gonopore
– Fertilization external
– Planktonic larvae
• Eggs may be trapped in female’s
tentacles and transferred to body
surface for larval brooding.
• Asexual
– Transverse fission and regeneration
Class Crinoidea
• Sea lilies and feather stars
• Most primitive living echinoderms
– Extensive fossil record
• Sea lilies
– Attach permanently to substrate by stalk
– Crown
• Calyx and arms with pinnules
• Mouth and anus open to upper (oral) surface.
• Feather stars
– Lack stalk
– Swimming and crawling
– Cling to substrate by cirri when at rest
Figure 16.14 A sea lily
Figure 16.15
A feather star
Class Crinoidea
• Maintenance functions
– Arms used in suspension feeding
• Plankton trapped by tube feet
• Transported with cilia to mouth
• Original function of water-vascular
system (?)
– Cup-shaped nerve mass with radial
nerves to arms
• Lack nerve ring
Class Crinoidea
• Reproduction and Development
– Many dioecious
– Others monoecious
• Protandry common
– Gametes released through ruptures in
walls of arms.
• Development
– Planktonic larvae metamorphose to adults.
• Some brood larvae on outer surface of arms
• Regeneration
– As in other echinoderms
Further Phylogenetic
• Crinoids most closely resemble oldest
– Mouth-up suspension feeding probably
original orientation and function of watervascular system
– Calcium carbonate endoskeleton may have
evolved to support filtering arms.
• Mobile, mouth-down, free-living lifestyle
probably secondarily derived
– Ampullae, suction disks, tentacles, and
secondary bilateral symmetry adaptations
for this mobile life style
Figure 16.16 Evolutionary relationships among the
echinoderm classes.
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