THE STARFISH 137 



opens aborally. The pair of lobed structures attached to the intestine 

 are the rectal caeca. Remove the aboral wall of the disk except for a 

 small piece around the anus. Leave all other structures intact. 



(c) Now remove the aboral wall from the two remaining arms of 

 the trivium, using the same precautions as before, but instead of cut- 

 ting off the tips of the arms, cut outward from the disk on each side 

 of the arms in such a way that the pyloric caeca will remain intact. 

 Remove also the aboral wall from the proximal third of the two re- 

 maining arms without destroying the madreporite. To get a better 

 view of the stomach, trim down the body wall in the angles between 

 the arms but do not injure either the parts of the wall to which the 

 reproductive organs are attached or the region beneath the madre- 

 porite. 



(d) Cut across the bases of the pyloric cseca in the two arms of the 

 bivium and, without destroying the ducts leading to the stomach, 

 allow all five pairs to float out in the water. The stomach will now 

 be seen to consist of two portions, an oral cardiac region and an aboral 

 pyloric region. The cardiac portion is everted through the mouth in 

 feeding. Where are the retractor muscles that withdraw this cardiac 

 region? What muscles could produce the eversion? A diagrammatic 

 figure of a vertical, longitudinal section through one arm and the disk 

 should be consulted if available. Using the figure indicated at the 

 close of Exercise 2 (i), or making a new figure, show the cut wall 

 as it actually appears over only a limited area along one side of an 

 arm; for the remainder, a double line will suffice. Draw into this 

 figure the digestive system, showing one pair of digestive glands in 

 detail and the others cut off near the base. Cut off the digestive 

 glands and free the cardiac region of the stomach by cutting all the 

 retractor muscles. Float out this region and determine its oral at- 

 tachment, after which it should be removed by cutting close to the 

 peristome. Before discarding, reexamine this central portion of the 

 digestive system and check your drawing of the parts. 



Exercise 5. — The Reproductive System. 



(e) In specimens collected during the breeding season the repro- 

 ductive organs are large and extend into the arms; at other times 

 they are small. Five pairs of gonads will be found in the ccelom at 

 the sides of the arms near the disk. How and where are these gonads 

 attached? Each is a branched, lobed sac from which a duct opens to 

 the outside through a perforated plate in the angle between neigh- 

 boring arms. What is the exact location of this opening? Starfishes 

 are either male or female, although the gonads have much the same 



