166 



GENERAL ZOOLOGY 



has developed no tentacles, but has little knobby buds 

 along its sides. Both sorts of individuals have only two 

 layers (ectoderm and entoderm) in the body wall. The 

 hydranth is a polyp (Fig. 68) which differs but little from a 

 gastrula, except for the connections it has with the other 

 individuals in a colony. 



Self -maintenance. The Obelia hydranths are the only 

 individuals in a colony capable of feeding. Each one 

 catches little animals in its tentacles and pokes them 

 through its mouth into the simple digestive cavity. There 



FIG. 69. A cnidoblast, or cell containing a nematocyst. A, with nematocyst 

 coiled; B, nematocyst discharged; c, cnidocil, or trigger. 



is no opportunity to move about to look for food, hence 

 Obelia always grows where there are currents and waves, so 

 that food may be brought to it. If the animals captured are 

 too lively and struggle about, they are stung by the nema- 

 tocysts which cover the tentacles. The nematocysts 

 of ccelenterates (Fig. 69) contain a poison which soon 

 kills small animals, so that they can be swallowed without 

 trouble. 



After the food enters the enteron, or digestive cavity 

 (Fig. 68, a) it is kept circulating by the movement of little 

 flagella on certain cells in the epithelial lining (= ento- 



