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.
Note jaw like structures know as pedicellaria. We will wait until viewing
the sea urchin in which these structures are highly visible and more mobile
to discuss their function.
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.
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, 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 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.
NO SEA CUCUMBERS SKIP 6
Sea urchins
Exercise 7: Behavior
Obtain a living sea urchin and place in a small dish under the dissecting scope.
Focus on the tube feet and see if you can observe the cycle of expansion and contraction
of feet that enable the animal to move.
Film tube feet in motion. Is there a rhythm to the motion and can you time regular
waver of contraction and relaxation?
Exercise eight: Pedicellaria in the sea urchin are found on the apical surface.
We have purple and tuxedo urchins. To locate pedicellaria on the tuxedo
urchin focus on what appears to be smooth area between the spines.
Various functions have been attributed to these structures. They have
implecated in defense as well as feeding. They have been observed cleaning
potential parasites and sediment off the animals. Pedicellaria of a few
species may contain venom. which aids in their defensive function. They
also have been observed catching small prey. Found in starfish and sea
urchins, two top predators (with the exception of a few herbivores) in all the
ecosystems they have been found, one wonders why they need to use these
for supplemental feeding!
Obtain a short film of pedicellaria movement . Also 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 Nine: 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.
Take a video of the aristotle's lantern in the sea urchins available.
At the end of this lab, you should be able to compare movement and feeding
in 3 echinoderms, sea urchins, sea stars/or brittle stars and sea cucumbers.
You should also be able to compare external anatomy that may be utilized in
one form versus the other in “circulation”, movement and feeding.