XV. CCELENTERATES 



The Ccelenterata, — The sponge, as we have seen, is an animal 

 of simple form and of a low order in the scale of life. Another 

 simple animal is the hydra — a type of the group Ccelenterata. 

 The group Coelenterata includes animals which have a common 

 food tube and body cavity. 



Hydra. — The common hydra lives in fresh water. It may be 

 found living on dead leaves, submerged sticks, stones, or weeds in 

 almost any fresh-water pond. In a small fresh-water aquarium, 

 they almost always 

 attach themselves to 

 the side of the jar, 

 where they can be 

 watched and their 

 movements observed.^ 



Notice that when 

 contracted thev re- 

 semble a little whitish 

 ball of jellylike sub- 

 stance. When undis- 

 turbed they elongate 

 into a hollow cylinder 

 attached at one end. 

 A small oval opening 

 is found at the free 

 end, surrounded with 

 a number of little wav- 

 ing arms called tenta- 

 cles. Count them to see how many there are. If by chance a 

 small water flea or other crustacean on which the hydra feeds 



> See Hunter and Valentine, Manual, page 151. 



195 



Budding hydra, as seen under the low power of a com- 

 pound microscope; B, attached end; B^, B^, buds; 

 M, mouth; T, tentacles. 



