226 



GENERAL ZOOLOGY 



The longitudinal muscle fibers in the tube feet then con- 

 tract and drive the contained water back to the ampullae. 

 When the scores of tube feet in the advancing arms of 

 a starfish contract they pull the body along and extend 

 again to renew their hold. The arms themselves, mean- 

 while, are turned up a little on the tips and remain in 

 a more or less set attitude. They may bend now and 

 then to pass an obstruction. When it is necessary the 



Interradial pouch 



of intestine 

 Sieve plate 

 Stone canal 



1 g 1 v - • 



Anus 



Intestinal csecum 



l 



Cavity of pyloric csecum 

 Pyloric caecum 



Calcareous plate j 

 Reproductive gland j 

 Nerve ring 



^mm^^pulla I h *?** «** 

 | ! i | Stomach pouch Tuhe foot 

 Retractor muscle of stomach 

 Mouth ' i Radial nerve 



Ring canal Stomach 



Fig. 123. Dissection of a starfish in oral to aboral section. 



(Slightly reduced) 



animal becomes quite flexible. It can bend its arms and 

 central disk and pass through seemingly impossible holes. 

 Digestive System. In the animals that have been studied 

 previously, the digestive system has been a more or less 

 complicated tube running straight through the body, or 

 sometimes coiled. The digestive system in the starfish is 

 entirely different (Fig. 123). There is a circular mouth 

 which leads directly into a large stomach filling most of 

 the space within the disk. Two short pouches, or pockets, 

 extend from the stomach into each arm, thus giving more 

 space for digestion. From its aboral end the stomach 

 gives off five branches, one to each arm. Each of these 



