STARFISH LAB

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7TH GRADE SCIENCE

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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