76 MANUAL OF ANIMAL BIOLOGY 



the groove can be entirely closed and protection thus offered to the 

 enclosed tube feet. 



B. Organ Systems 



1. Nutritive System 



The nutritive system of the Starfish, conforming to the general 

 structural plan of this unique organism, is quite unusual in various 

 structural features. The mouth, as has been noted, is centrally 

 placed on the under surface of the central disc and opens into a short 

 tube, the esophagus. The latter continues upward (aborally) and 

 leads into a large, five-lobed, thin-walled sac, the stomach, which 

 occupies most of the space in the central disc. The oral, or cardiac, 

 portion of the stomach is so constructed that it can be pushed out, 

 or everted, through the mouth opening when the animal is feeding. 

 From the somewhat smaller pyloric portion of the stomach, which 

 lies above the cardiac portion, five pairs of pyloric caeca arise ; 

 each pair extending into and throughout the entire length of an 

 arm. The caeca branch extensively to form a great number of 

 lateral pouches in which the digestive enzymes are secreted. A 

 tiny intestinal tube, the rectum, also has its origin in the pyloric 

 portion of the stomach from which it continues to the very small 

 anal opening on the aboral surface of the central disc. 



When the animal feeds, the cardiac portion of the stomach is 

 everted through the mouth opening, and, wrapping itself around the 

 food material, proceeds to digest it outside the body. Indigestible 

 materials taken into the digestive system are later egested via the 

 mouth so that very little, if any, use is made of the intestine in this 

 connection. The digested food is absorbed by certain of the cells 

 lining the alimentary tract, then passed on to the outer cellular 

 layers, and finally transferred to a body fluid (coelomic fluid) 

 present in the body cavity (coelom). The latter surrounds the 

 stomach and continues into each arm, so that the food materials 

 picked up by the coelomic fluid are distributed to all parts of the 

 organism. The movement of the fluid throughout the body 

 spaces is more or less irregular, but it is aided by the action of cili- 

 ated cells present in certain areas of the lining of the coelom. 



2. Water Vascular System 

 The echinoderm water vascular system is unimitated, nothing 

 comparable being found in any other group of animals. It consists 

 of a complex system of closed tubes which may be said to begin 



