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. 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. Use Asterina (small) species.
Obtain a specimen, place in a small dish and add a few (one maybe two)
fish flakes to the dish near the animals. 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.. Does the animal’s body move in any
way to accommodate feeding? Are the arms involved? Can you see food
moving into the mouth?
Exercise two: External Anatomy
Right the animal.
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 oralaboral 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 oblong 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 .
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.
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.
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. 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
yellowish-orange 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 . 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.
Ophiuroidea
Also known as brittle stars or serpent stars, ophiuroids have long sinuous
arms sharply demarcated from the central disk. The arms contain vertebralike 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. The larva is the ophiopluteus.
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 anterior-posterior, dorsal-ventral, right-left are not
applicable.
Exercise four: Treat a brittle star as the sea star Watch the animals
move and compare their movements with that of a sea star in your
journal. Next obtain a dish that contains brittle stars on live rock.
How do their movements compare with a brittle star moving freely in
water and away from you? Record your observations in your notebook
Go back to your specimen in the dish not containing any rocks and
examine its external anatomy.
Aboral Disk Surface
Look at the aboral surface of the disk and arms with
magnification. Ophiuroids have no papulae, no pedicellariae, and no
paxillae such as are common in asteroids. The epidermis is a thin, nonciliated 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 those of Ophioderma are covered by
dermal granules and cannot be seen. There are no distinct muscle layers in
the body wall of ophiuroids.
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. 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,
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.
Exercise five: Take a photograph of the oral surface and label as many
structures as you can. 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.
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 . One of them is a tiny
bit larger than the others and bears the opening of the madreporite, which
you probably will not see.
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. In Ophioderma each bursa has two apertures although some species
have only one long slit beside the base of the arm.
Arms: Do your best but realize brittle stars do not like light. You may
be able to see more in specimens crawling around the sponge.
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 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?
Aboral surface of the arm of Ophioderma brevispina.
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, 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 perivisceral 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 six: Extra points will be awarded to those groups able to
furnish the class with great arm shots.
Holothurians, sea cucumbers.
The most important feature distinguishing the sea cucumbers is a calcareous
ring that encircles the pharynx or throat. This ring serves as an attachment
point for muscles operating the oral tentacles and for the anterior ends of
other muscles that contract the body longitudinally. Oral tentacles may be
simple, digitate (with finger-like projections), pinnate (feather-like), or
peltate (flattened and shield-like). A third key feature, found in 90% of
living species, is the reduction of the skeleton to microscopic ossicles
(Figure 1). In some species, the ossicles may be enlarged and plate-like.the
holothurian water vascular system consists of an anterior ring canal from
which arise long canals running posteriorly (not shown in Figure 2). Despite
their similarity to the radial canals of other echinoderms, these latter
structures arise embryologically in a quite different manner. For this reason
these canals in holothurians have been recently renamed longitudinal canals
(Mooi and David 1997). In holothurians, the larval structures that would
form the radial canals in other echinoderms instead become the five primary
tentacles. Also, holothurians with the exception of members in Elasipodida
have a madrepore that opens into the coelom (body cavity). In contrast,
elasipodans and nearly all other echinoderms have a madrepore that opens
externally.
Exercise seven: Observe and describe in your journal how the sea
cucumbers in the tank use their arms in feeding.
Sea urchins and Pedicellaria
Pedicellaria in the sea urchin are found on the apical surface. We have a
pencil urchin and two tuxedo urchins. To locate pedicellaria on the tuxedo
urchin focus on what appears to be smooth area between the spines. Those
looking for pedicellaria on the pencil urchin deserve extra credit. Be sure to
call the rest of the class over when you find them so they can also observe
their location. Can you or others in the class try to speculate on why they
are found in this location on pencil urchins?
Exercise seven: Obtain a short film of pedicellaria movement in tuxedo
urchins or an photograph of pedicellaria in the pencil urchins. Compare
the spine found on the sea urchins and starfish in your journal. Are the
spines for example, more motile in the sea urchin?
Exercise eight: Sea urchins and Aristotle’s lantern,
Before returning your specimen to the larger dish turn it over on its distal
side . Can you locate Aristotle’s lantern or the “jaws” used by the sea urchin
to feed? These jaws are fairly large and capable of crushing and tearing
apart shells and other protective layering of prey (including other
echinoderms) and the reason why most sea urchins are not considered reef
safe. Fortunately, the Tuxedos are one of the few species that only feeds on
algae.
Exercise eight: Describe or include a photograph of an aristotle's
lantern in the sea urchins available.