Download Echinoderms

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 chocolate stars (large) or the Asterina (small) species.
(Do not use the blue linckia or sand sifting star for these exercises.
The first is a vegetarian and the latter will not feed unless buried. They
are in the lab simply for you to observe.)
Obtain a specimen of one of the species, 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, try the
smaller species that will almost always feed upside down on fish food. 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 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
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.