SIMPLE, MANY-CELLED ANIMALS; HYDRA 315 



If tiny bits of meat are dropped near them, the tentacles 

 direct the meat to the mouth. But food does not go 

 directly into the cell, as it does in one-celled animals. 

 We can see that this is not necessary, for the hydra's 

 body and tentacles are made up of two rows of cells. 

 The inner cells have the duty of digesting food, while the 

 outer cells serve to protect them. The digested food is 

 passed through the cell walls, from the inner cells to the 

 outer ones. The tentacles do not simply push food 

 toward the mouth, but some of their outer cells have 

 stinging weapons for paralyzing the hydra's prey. 



Often the hydra may be seen budding, as yeast does 

 (cf. Fig. 103, 129). The young hydra remains attached 

 to the older one for a time, and then separates from it. 

 If a hydra is cut in two, each half produces the necessary 

 cells, and forms a new creature. The hydra is also re- 

 produced by a sexual method. There are not two sexes, 

 but some of the cells produce egg-like 

 bodies, and other cells on the same hydra 

 produce swimming cells that fertilize the 

 egg-like cells. When the two kinds of 

 cells unite, they form an embryo hydra. 

 This falls to the bottom of the water, and 

 in time develops into a full grown animal. 



More highly developed forms of the hydra 

 have a great number of animals in a colony (Fig. 

 257) . The colony has root-like ' ' holdfasts, ' ' and FlG - 2 Coral 

 attaches itself to rocks. Hydra-like animals are 

 called polyps, from words meaning "many feet." The coral polyp 

 is a common form. Each coral polyp takes limestone from sea water, 

 and forms a hard internal structure (coral), which remains when the 



