3. On the abo'ral surface of the disc, near the junction of two 

 of the arms is a small, conspicuously colored, circular body, 

 the madrcporic plate. The two arms adjacent to this plate are some- 

 times referred to as the "bivium" and the remaining three as the 

 "trivium". The radial symmetry of the animal is disturbed ex- 

 ternally only by the madreporic plate. Examine this plate with a 

 lens and determine its structure. 



4. On the oral surface is the mouth. Xote its size and see if it 

 is provided with jaws of any sort. 



5. Radiating from the mouth are the ambulacra! grooves, one on 

 each arm. In these grooves are the ambulacra!, or tube feet. Do 

 they have a definite arrangement ? Along the sides of the grooves 

 are slender spines which differ from those covering the general 

 body in being movable. 



6. Scrape the tube feet from a portion of an ambulacral groove 

 of a dried specimen and notice the pores through which the feet 

 pass to organs within the arm. Notice also the exposed ambulacral 

 plates, and determine their relation to the pores. 



Draw figures of the oral and aboral surfaces of a starfish and a 

 diagram to show the relation of the ambulacral plates and pores. 



7. On the aboral surface of the body find the dermal branchiae, 

 folds of the soft part of the body-covering, extending out between 

 the calacarious plates, which give the body its rigidity. Look with 

 a hand lens for small two- jawed structures around the bases of the 

 spines. Remove some of these pedieellaria and examine under the 

 microscope. 



Draw a pedicellarium. 



B. INTERNAL STRUCTURE. 



Make the dissection under water and in cutting through the 

 integument be careful not to injure the underlying soft parts. 



With scissors cut through the aboral wall near the tips of the 

 rays of the trivium. Carry the cuts along the sides of the 

 rays to the disk. Lift up the integument at the tip of each arm 

 and carefully cut away the mesenteries which attach the organs to 

 it. Cut the membranes which extend into the disk opposite the 

 junction of the arms, and remove the three rayed flap of integument 

 thus freed, cutting as close as possible to the madreporite, but 

 leaving this in place. 



I. DIGESTIVE SYSTEM. 



In studying this system you should constantly bear in mind the 



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