frog dissection


                           The phylum Echinodermata includes starfishes or 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 A. vulgaris of the Atlantic coast and
                           Pisaster ochraceus of the Pacific coast.
                           PURPOSE:  To study the internal and external anatomy of a
                           MATERIALS:  A preserved specimen, dissecting pan, scalpel or
                           razor blade, probe, hand lens
                           Kingdom  - Animalia
                           Phylum   - Echinodermata
                           1. EXTERNAL DISSECTION
                           A. Study a fluid-preserved specimen in a pan of water and
                           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
                           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
                           With the starfish in water and the aboral surface uppermost,
                           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
                           Intestine - very slender, short, from pyloric stomach to 
                           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:
                                1. Stone canal - limy tube in an angle of bivium, from 
                                     madreporite to ring canal.
                                2. Ring canal - hard, circular, around mouth region
                                3. Tiedemann bodies - nine, small swellings in ring canal
                                4. Radial canal - from ring canal along each arm, see cross
                                     section; connects by transverse canals to ampullae.
                                5. Ampullae - many, small, spherical, in floor of coelom -
                                     connect to tube feet
                                6. Tube feet
                                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?  In adhering to solid

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