INTRODUCTION:
The phylum Echinodermata includes starfishes (sea stars), brittle stars, sea urchins, sea lilies, and sea cucumbers. All but the last have a limy internal skeleton and hard external spines or plates. They are fixed or slow-moving inhabitants of the sea, from the high-tide zone to considerable depths. Often they are abundant but none form colonies. Species of shallow water are easily collected by hand at low tide and deeper ones are captured by dredging. Those with skeletons are easily prepared merely by drying but specimens for dissection are preserved in formalin or alcohol. Eggs of starfishes and sea urchins can readily be obtained in quantity and fertilized as needed; hence, they serve for study in embryonic development and in many experimental researches on animal eggs.
Common species of starfishes used for class work are Asterias forbesi and Asterias vulgaris of the Atlantic coast and Pisaster ochraceus of the Pacific coast.

PURPOSE:
To study the internal and external anatomy of a Echinoderms.

MATERIALS:
A preserved specimen, dissecting pan, scalpel or razor blade, probe, hand lens.

CLASSIFICATION:
Kingdom - Animalia
Phylum - Echinodermata

1. EXTERNAL DISSECTION:
A. Study a fluid-preserved specimen in a pan and identify:
1. Arms or rays - projecting from disc
2. Central disc - poorly defined
3. Oral surface - usually concave
4. Aboral surface - exposed in life
5. Madreporite - small white circular area, off-center on aboral surface of disc
6. Anus - minute, centered aborally on disc
7. Bivium - the two arms close to the madreporite
8. Spines - many short, rough, limy, in patterns over aboral surface
9. Eyespot - small, pigmented on one end of each arm
10. Ambulacral grooves - one along oral surface of each ray
11. Ambulacral spines - slender rods on margins of ambulacral grooves
12. Tube feet - soft, slender, with expanded tips; 2 or 4 rows in each groove
13. Tentacle - soft, on end of each arm

B. Examine a small area on the aboral surface under a binocular microscope and distinguish the following:
1. Papulae or dermal branchiae - thin hollow soft projections which function as gills
2. Pedicellariae - minute pincers with two jaws; in circles around spines and elsewhere

2. INTERNAL DISSECTION:
Place the starfish with the aboral surface facing you and use stout scissors to cut off the extreme tip of each arm of the trivium. Then cut along the sides of these three arms. Use care not to injure any internal organs. In turn, lift and carefully remove the aboral surface of each arm, loosening the delicate mesenteries beneath by which the soft organs are attached. Also, cut around the disc (but not the bivium) and remove the aboral surface, leaving the madreporite in place. Finally, cut transversely, at mid-length, through one arm of the bivium to provide a cross section. Identify:
Coelom or body cavity - space containing internal organs; lined with thin ciliated peritoneum.
Stomach - disc, thin, sac-like, and 5-lobed, cardiac portion, larger, with pleated walls and retractor muscles; pyloric portion, aboral, smaller, 5-sided and smoother.
Intestine - very slender, short, from pyloric stomach to anus
Hepatic caeca - a pair in each arm, greenish, long, of many finger-like lobes, each caecum with duct to pyloric stomach; also termer digestive glands, liver, or pyloric caeca.
Gonads - in each arm, below hepatic caeca, bilobed; each attached by duct opening aborally; sexes separate.

3. WATER VASCULAR SYSTEM:
Remove the side of the stomach near the madreporite; then starting from the latter, trace the parts of the system. If available, examine a demonstration specimen having the system injected with colored mass. Identify the following structures:
Stone canal - limy tube in an angle of bivium, from madreporite to ring canal.
Ring canal - hard, circular, around mouth region
Tiedemann bodies - nine, small swellings in ring canal
Radial canal - from ring canal along each arm, see cross section; connects by transverse canals to ampullae.
Ampullae - many, small, spherical, in floor of coelom - connect to tube feet
Tube feet -

Questions:
What is the mode of action of the water vascular system?
How do the ampullae and tube feet act to affect locomotion?
How do the tube feet serve in food taking?
How do the tube feet serve in adhering to solid objects?