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Echinoderms are secondarily radially symmetric deuterostomes whose ancestors
were bilaterally symmetric. The adult radial symmetry is pentamerous with body
parts occurring in fives or multiples thereof. Echinoderms have strong affinities
with the ancestral trimeric deuterostomes especially in the tripartite organization of
the coelomic cavities. Echinoderm larvae have the coelom divided into three
regions, as is typical of the early coelomates, and these regions have important
adult derivatives. All echinoderms are marine and benthic. About 6000 Recent
species are known but the fossil record includes 13,000 extinct species.
An important echinoderm characteristic is the water vascular system that in most
groups functions in support of locomotory tube feet but is also important in gas
exchange, excretion, and feeding. The body wall includes a thick connective tissue
dermis in which calcareous ossicles (little bones) are almost always an important
component. These ossicles make up an endoskeleton which assumes different
forms in different taxa. In most echinoderms calcareous spines of various sizes
and shapes arise from the dermis and extend from the body surface and are alluded
to by the name echinoderm (= spiny skin). The connective tissue is mutable and its
consistency is under nervous control.
Excretion in echinoderms is accomplished by simple diffusion of metabolic
wastes (ammonia) across thin permeable regions of the body wall. A variety of gas
exchange structures, including the tube feet, is found in various echinoderms. A
hemal system is present but its role in transport is still poorly understood and the
chief transport system is the circulating fluid of the various coelomic
compartments. The hemal system may be through transport system that delivers
nutrients from the gut to these compartments for local distribution. The nervous
system consists of two central intraepidermal nerve rings from which arise radial
nerves to the periphery. Echinoderms are gonochoric and fertilization is usually
external.
This week in laboratory, you will start your comparative study of echinoderms.
One individual should read the text to a partner who trys to find the structures
described. Be sure to switch duties often. Next week you will be expected to be
familiar with the structures you will be looking at this week and do more
videotaping of echinoderms. So please take the time to find and learn the
basic morphology of echinoderms this week. Your photographs should be
good enough to serve as comparisions to those you will obtain next week.
Exercise one to three: Asteroidea
Asteroids are the sea stars, which are the best known echinoderms. Sea
stars usually have five arms, but sometimes more, radiating from a central disk.
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The ossicles of the body wall are rodlike and articulate via fibrous junctions to
form a flexible grid. Respiration is with the tube feet and papulae. Each arm has
an eyespot at its tip. A pair of large pyloric ceca and a pair of gonads are present in
each arm. Gastric hemal tufts are present. About 1500 Recent species are known.
Exercise one: Try to film feeding in one of the species available of sea stars
Obtain a specimen of one of the species available, place in a small dish and
add a few (one maybe two) fish flakes to the dish near the animals for the
small species and one small fish for the larger species. You may have to place
the larger species on the fish to encourage feeding. Wait till the animals start
to feed and then start your observations. Once they start actively feeding, turn
the animal upside down and see if you can see exactly how the animal feeds. The
larger species may invert their stomach while feeding.. If the large species is
uncooperative, we may have some smaller species that will feed upside down on
fish food.
Exercise two: External Anatomy
The body is divided into a central disk from which radiate five arms. The
principal body axis, and the axis of symmetry, is the short oral-aboral axis, which
passes vertically through the center of the disk. The animal's pale lower side is the
oral surface and structures on this side are said to be oral. The dark upper side is
the aboral surface and structures on this side are aboral. Structures remote from
the axis are said to be peripheral whereas those near the axis are central. In
radially symmetrical animals anterior-posterior, dorsal-ventral, right-left are
irrelevant and have no meaning.
Aboral Surface
Find the calcareous, usually orange or brown madreporite on the aboral
surface of the disk Examine it with the high power of the dissecting microscope
and note its grooved surface. Numerous microscopic pores in the bottoms of the
grooves open into canals (stone canal and axial canal) of the internal water
vascular system .
Orient the star with the aboral side up and with the madreporite close to
you. The arm on the left of the madreporite is arm I, arm II is to the right of the
madreporite, and the remaining arms are numbered sequentially moving
counterclockwise around the star .
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Aboral view of Asterias.
The surface is covered by a monociliated epidermis. Can you see evidence of such
under the stereoscope?
On the aboral surface notice the numerous small fixed spines, so-called because
they are fixed in position and cannot move. These spines are extensions of the
calcareous endoskeleton in the body wall. Gently push one of the spines with the
tip of a needle to see if it moves. Look closely at the spines with the highest
magnification of the dissecting microscope and confirm that they are indeed
internal and are covered by a thin layer of living tissue, the epidermis.
You may also be able to see circle of short-stemmed, white pedicellariae
(singular: pedicellaria), around the spine.. Pedicellariae have an endoskeleton of
ossicles . These can move and in some species have jaws that open and close.
Can you photograph or film the pedicellaria found in the species available.
Do they have movable jaws?
See if you can see soft, thin-walled, translucent, fingerlike papulae between the
spines as well. Papulae are thin-walled diverticula of the coelom through the body
wall and are its respiratory organs. The ciliated peritoneum generates a
bidirectional flow of fluid into and out of the papulae. The papulae are muscular
and can be retracted into the surface of the body wall. They may be retracted and
inconspicuous in somespecimens. Touch a papula with the microneedle and
observe its response.
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The anus is located near the center of the aboral surface but is almost impossible to
demonstrate externally. It is surrounded by a palisade of tiny ossicles, much
smaller than the spines that stud the surface of the disk and is in an area free of
papulae.
Exercise two a: Photograph the aboral surface or in large specimen the area
aroung madreporite. Label the madreportie and spines.
Oral Surface
Turn the animal over and study the oral surface. Find the large mouth in the center
of the disk, surrounded by the thin peristomial membrane . The yellowishorange curtain-like folds of the cardiac stomach may be visible inside the mouth.
Five deep ambulacral grooves radiate outward from the mouth, one along the
midline of the oral surface of each arm . Each groove lies on an ambulacral axis.
The numerous soft, tubular structures projecting into the groove from either side
are the tube feet, or podia. Two rows of tube feet are present on each side of the
groove. The tube feet of Asterias bear suckers at their distal ends. Note the rows
of long, flattened movable spines on each side of the ambulacral groove. The
word ambulacrum is Latin for "covered way," an apt name as these spines are used
to cover the groove to protect the tube feet.
Look at the tip of one of the arms . As is usual in radially symmetrical animals, the
sensory structures are arrayed around the periphery, which in sea stars is the tips of
the arms. Several long, narrow sensory tube feet extend from the tip of each arm.
These are easily seen in living specimens but contract and become inconspicuous
in preserved material. They have chemo- and mechanoreceptors. At the tip of the
arm is a small circle of short, blunt movable spines that are not associated with
pedicellariae. These spines surround a small, pale red or yellow eyespot. The
eyespot is on the oral surface of the arm, almost at the tip.
Exercise three: a. Take a photograph of one of the arms at high power and
label what your can. Turn the animal over and watch it move, which it will
want to do and quickly away from you. B. Describe the movement in your
journal.
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Exercise three and four: Ophiuroidea
Also known as brittle stars or serpent stars, ophiuroids have long sinuous arms
sharply demarcated from the central disk. The arms contain vertebra-like ossicles
and are very flexible, exhibiting a motion reminiscent of that of snakes (ophio =
snake). About 2000 living species are known.
The ophiuroid gut ends blindly, there being no anus or intestine. Digestion and
absorption occur in the stomach and pyloric ceca are absent. The digestive system
is confined to the central disk with none of it extending into the slender arms. The
madreporite is oral and the tube feet lack suckers. The gonads open into
invaginations of the integument which also serve as respiratory organs.
Brittle stars are not as cooperative as sea stars and are delicate animals. Our
specimens are already showing signs of stress. Try never to expose them to
air as you transfer specimens from one container into another. Also try to be
very gentle with them, ao both classes have good experiences with the
specimens we have. Use the handles, instead of the tips of forceps or teasing
needle to poke and prode them.
Exercise three, movement: Treat a brittle star as the sea star a. Watch the
animals move and compare their movements with that of a sea star in your
journal. If your animal is reluctant to move freely, shine a bright light on the
animal. Brittle stars are noctornal and avoid light. Next give your specimen a
rock to hide and explore. b. How do movements over a rock compare with a
brittle star moving freely in water and away from you? Record your
observations in your notebook.
Exericise 4. External Anatomy
The dark upper side of Ophioderma is the aboral surface and the pale side
opposite it is the oral surface. Note the well defined central disk (Fig below)
from which extend five long snakelike arms (ophio = snake.) The aboral-oral
axis runs through the center of the disk perpendicular to the surfaces and is the axis
of radial symmetry. Structures remote from the axis are said to be peripheral
whereas those near the axis are central. In radially symmetrical animals anteriorposterior, dorsal-ventral, right-left are not applicable.
Aboral Disk Surface
a. Take a photograph of the animal’s central disk. The following description
of the aboral surface should help you locate important structures.
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Examine the aboral surface of the disk and arms undre low and high
magnification. Ophiuroids have no papulae, no pedicellariae, and no paxillae such
as are common in asteroids. The epidermis is a thin, non-ciliated syncytium and
most of the body wall is a connective tissue dermis, which contains abundant
calcareous ossicles of many sizes and shapes and are often known as shields.
The aboral disk surface is covered by small, spherical, calcareous dermal
granules, which are themselves covered by epidermis In many genera the aboral
disk bears larger, distinct, scalelike ossicles. The granules are best seen at about
40X magnification. At the base of each arm, but on the disc, are two large oval
radial shields . Radial shields are an obvious feature of the aboral disks of most
ophiuroids but soma are covered by dermal granules and cannot be seen. There are
no distinct muscle layers in the body wall of ophiuroids
Aboral surface of the arm of Ophioderma brevispina.
Most of the large dermal ossicles, or shields, covering the aboral surface of the
disk are hidden by dermal granules but the shields of the arms are exposed and
easily observed. Most important are the four rows of overlapping dermal shields
on each arm. A row of aboral arm shields runs along the top of the arm and
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similar oral arm shields extend along the bottom. Each side of the arm bears a
row of lateral arm shields. Thus the arm is encircled by a linear series of rings,
each consisting of four shields. The name "ophioderma" means snake skin. Is the
name apt?
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b. Take a photograph of the oral surface and label as many structures as you
can. Label at least the mouth and tube feet. The diagram and descriptions
below are provided to help you. Try to see as much as possible in the leg
region. Brittle stars are not as cooperative as sea stars and may quickly turn
over. Again be patient with these beautiful animals, they break off legs easily
and often develop fatal infections if they do. The following description should
help you locate important structures.
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Oral Disk Surface
Turn the animal over and examine the oral surface of the disk with magnification.
Unlike that of the aboral disc, the epidermis of the oral disk is ciliated.
Find the star-shaped mouth at the center of the oral disk . Its margins are formed
by five triangular, calcareous jaws, which protrude from the sides into the mouth.
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Such jaws are characteristic of ophiuroids and bear teeth which, like the jaws, are
specialized ossicles, on their inner margins. You can see them clearly by looking
into the open mouth. If the mouth is closed, gently use fine forceps to push two
opposing jaws apart. The mouth opens into a large stomach which occupies most
of the space inside the disk, and which can be seen if the mouth is open.
The oral surface of the disk of Ophioderma
Five large oval ossicles, known as oral disc shields, are located on the surface of
the oral disk, one at the base of each jaw.
Several pairs of large, soft buccal tube feet (= buccal podia), are associated with
the mouth. They lie in the gap between adjacent jaws
The five pairs of genital bursae are invaginations of the oral surface of the disk.
The exhalant bursal slit is closest to the mouth and the inhalant bursal slit is
peripheral, farther up the side of the disk . In species with only one slit the
peripheral end if inhalant and the central end is exhalant.
Arms: Do your best but realize brittle stars do not like light.
About seven short spines extend from the free margin of each lateral shield and are
responsible for the specific name "brevispina" (=short spines). These spines are
short in Ophioderma brevispinum but much longer in most ophiuroids. A tube foot,
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or tentacle, arises at the oral end of each row of spines. A pair of small tentacular
spines protects the base of each tube foot, or podium. Ophiuroid tube feet are
sometimes called tentacles. They do not have suckers and their ampullae are
tiny. Touch a suspected tube foot with a microneedle. If it is soft and flexible it is
a podium. If hard, then it is a spine.
Oral surface of the arm of Ophioderma brevispina.
Inside each arm is a chain of large, blocky vertebra, or vertebral ossicles . These
are homologous to the ambulacral ossicles of seastars but they have been
internalized and modified to function like vertebrate vertebrae. In effect they are
the axial skeleton of the arm. They occupy most of the cross sectional area of the
arm. The coelom of the disk extends into the arms as a small canal between the
vertebra and the aboral arm shields.
The radial canal of the water vascular system runs below the vertebra and above
the oral arm shields. You can see the space through which it runs but the canal
itself is probably not visible.
Exercise 5: Holothuroidea
The holothuroids, or sea cucumbers, are echinoderms in which the body is
elongated along the aboral/oral axis. The body wall is thick and well developed. It
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consists of a nonciliated epidermis, connective tissue dermis, circular and
longitudinal muscles, and a ciliated peritoneum. It is unusual in that its
longitudinal muscles are in five longitudinal bands and the ossicles are microscopic
plates scattered in the dermis. Abundant collagen fibers in the mutable dermis can,
under nervous control, be cross-linked with each other to form a rigid skeleton that
can scarcely be cut with a scalpel.
The gut is a simple tube extending from mouth to anus. Gas exchange is via
diverticula of the posterior gut. Large branched tube feet encircle the mouth and
are used for feeding. The hemal system is better developed than is typical in
echinoderms and the coelom is large and spacious. The madreporite opens into the
perivisceral coelom, rather than to the outside. Sea cucumbers are gonochoric and
have a single gonad opening to the exterior via a gonopore at the oral end.
External Anatomy
Symmetry
The body is elongate with the mouth at the oral end and the anus at the opposite,
aboral, end. A weak bilateral symmetry is superimposed on the underlying
echinoderm radial symmetry. The aboral-oral axis is the long axis of the body and
is the axis of symmetry, both bilateral and radial. Cucumbers are radially
symmetrical but also possess a superficial bilateral symmetry in which the oral end
is also the anterior end. The aboral end is also the posterior end.
Introvert
The anterior end bears mouth and a circle of ten branched tentacles, the buccal
podia. It is an introvert that can be fully retracted into the body by a set of
powerful retractor muscles. Threatened animals or those preserved without
adequate relaxation may have retracted introverts, in which case the tentacles and
mouth will not be apparent externally. Try to tempt feeding in your specimens by
adding some microfood to the dish. DO NOT OVERFEED.
Exercise five: a. Obtain if at all possible a video of feeding in your cucumber.
Most cucumbers creep over the surface of the substratum or burrow into it. A
(very) few swim. When creeping they keep the ventral surface in contact with the
bottom and this surface is specialized for the purpose. The flat ventral
surface has better developed tube feet, which are known as locomotory
podia. They usually have suckers.
The specimens we have obtained are burrowers. How can you tell? Hint:
Compare their movements and the lack there of in the aquarium specimens
Also, where are those tube feet? b. Record your observations in your
journal.
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