33 6 Descriptive Zoology. 



Circulation. There is no well-developed circulatory 

 system, such as we find in the higher animals. There are 

 blood corpuscles in the liquid contained in the general body 

 cavity. This liquid pervades all parts, and is set in motion 

 by the cilia of the lining membrane, and by the general 

 movements of the stomach and the bending of the rays ; 

 these movements seem to suffice to circulate the contained 

 liquid, which probably directly receives the absorbed prod- 

 ucts of digestion. 



Respiration in Starfishes. There is no very complete 

 system of respiration in starfishes. In fact, no such system 

 is needed, for the whole body, inside and out, is constantly 

 bathed in sea water. Still, it is thought by many authors 

 that the tube feet are the chief agents in absorbing oxygen 

 from the water and giving off the waste. There are also 

 many holes through the aboral wall, from which extend 

 slender projections of the thin, soft lining membrane of 

 the body cavity. These are now supposed to be gills. 



The Nervous System and Senses. The nervous system 

 is near the surface, and can be seen without dissection. 

 Around the mouth is a five-angled nerve ring, which gives 

 off a radial nerve along each ray. It may readily be seen 

 by separating the tube feet along the middle line. 



At the extreme end of each ray is an eye spot, which 

 shows as a distinct red spot in a fresh specimen of the 

 common starfish. In alcoholic specimens it is hard to see. 



Close to the eye is what appears to be a tube foot, but 

 without the disk at the end. This is called a tentacle, and 

 is now believed to be an organ of smell. The sense of 

 smell seems to be a much better guide than sight in bring- 

 ing the starfish to its food. There is undoubtedly some 

 sensitiveness to touch, but of this and any other senses 

 little is known. 



