FRESH-WATER POLYP AND SOME ALLIES 269 



endoderm, and the outer one, the ectoderm. Cells of the 

 endoderm secrete fluid for digesting food, and even take 

 up small particles of food, digesting them inside the cell 

 substance. The ectodermal cells are modified into nettle 

 cells, nerve cells, muscle cells, and general surface cells. 



. Ectodermal cells 

 - Endodermal cells 



Older bud 



Forming bud-- 



Spermatozoa forming 

 r. . . Gastrovascular cavity 



Egg forming 



Fig. 140. Longitudinal section of a Hydra. (Much enlarged to 



show structure) * 



In capturing food (Fig. 139) the hydra makes use of very- 

 effective organs called nettling cells (Fig. 141). These organs 

 are found only in the phylum to which the hydra belongs. 

 By the aid of the nettling cells the prey is paralyzed or 

 killed, and then the tentacles push it into the mouth. 

 The nettling cells, which are of several different sizes, are 



1 Modified from Parker's Elementary Biology. 



