7TH GRADE SCIENCE

STARFISH LAB



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STARFISH DISSECTION

INTRODUCTION:
                                                      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
                                                      starfish
                                                      
                                                      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 of water 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
                                                      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
                                                      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:
                                                      
                                                           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
                                                      objects?
                                                      
                                                      

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