268 



GENERAL ZOOLOGY 



The small individuals are generally the youngest; these 

 have four tentacles. The larger, older ones gain in the 

 number of tentacles, up to the extreme number of eleven ; 

 but there is a greater percentage of individuals with six 



tentacles than with any 

 other number. 



Internally, the hydra is 

 the simplest animal we 

 have studied. There is no 

 distinct alimentary canal 

 separate from a body cav- 

 ity. Those animals that 

 do not have a body cavity 

 and an alimentary canal 

 have but one cavity to 

 take their place ; this cav- 

 ity is called the gastrovas- 

 cular cavity, because in 

 the same space food is di- 

 gested and carried about. 

 The fact that food and 

 oxygen can pass into any 

 portion of the body, even 

 into hollow tentacles, 

 accounts for the absence 

 of a circulatory system, 

 which usually serves the 

 purpose of transporting 

 oxygen, digested food, and also waste products from part 

 to part in the animal body. The mouth (Fig. 140) opens 

 into a cylindrical cavity with no indication of gullet. The 

 cavity extends by slender tubes out into each tentacle, and 

 in all parts only two layers of cells separate it from the 

 water outside. The inner one of these two layers is the 



Fig. 139. Hydra capturing a fish. 

 (Enlarged) 



Courtesy of the American Museum of 

 Natural History 



