ASTERIAS 215 



madreporite, but leaving this in place, remove the three-rayed 

 flap of integument thus freed. 



Irving, L.: Ciliary Currents in the Starfish. Jour. Exp. Zool., vol. 41, 

 1924. 



: Regulation of the pH Concentration and Its Relation to Metab- 

 olism and Respiration in the Starfish. Jour. General Physiology, 

 November 20, 1926. 



Kindred, J. E.: The Cellular Elements of the Perivisceral Fluid of 

 Echinoderms. Biol. Bull., vol. 46, 1924. 



Digestive System. — In studying this system you should 

 constantly bear in mind the peculiar method by which the 

 animal feeds, as the digestive system is highly modified to 

 suit this method. 



1. The short, cone-shaped intestine and the intestinal 

 caeca were probably removed with the integument. The in- 

 testine probably does not function, and may be regarded as 

 a vestige. It opens near the center of the disk, on the aboral 

 side, by a very minute anus that is very hard to see. 



2. The stomach, which occupies the greater part of the 

 space in the disk, is composed of a small aboral portion, the 

 pyloric division, that receives the ducts from the hepatic 

 caeca, and a larger, lobed, cardiac division, into which the 

 mouth opens. The cardiac portion may be everted through 

 the mouth, thus being turned wrong side out. Five pairs of 

 muscles, which draw this portion of the stomach back into 

 place, may be seen attached to the ridges formed by the am- 

 bulacral plates in each arm. How is it possible for the 

 stomach to be everted? What reason is there for two divi- 

 sions? 



3. In each arm is a pair of long, glandular organs, the so- 

 called hepatic caeca. The ducts of each pair unite and join 

 the pyloric division of the stomach by a common duct. These 

 are digestive glands. What reason is there for having ten 

 enormous digestive glands? Does this have anything to do 

 with the method of feeding? 



Make a drawing of the digestive system of the disk and 

 one arm. 



