HYDRA 61 



type of nematocysts just described, which paralyze the prey with a 

 poisonous substance, there is another smaller type which shows the 

 same general structure, but lacks the poison. The coiled, fila- 

 mentous tubes of this type of nematocyst curl around any small 

 projecting part of an animal which the Hydra is attempting to 

 capture, and in this way hold it firmly so that it can be conveyed to 

 the mouth. 



In addition to the epithelio-muscular cells and interstitial cells, 

 there are also developed from the ectoderm a considerable number 

 of nerve cells which are scattered among the epithelio-muscular 

 cells, and are also found lying deeper in the endoderm layer. The 

 bodies of these neurons of Hydra have become somewhat modi- 

 fied in their structure from that of a typical cell in that each pos- 

 sesses processes which connect with other nerve cells and thus 

 form a widely distributed nerve net. The latter constitutes a 

 primitive type of nervous system equipped to receive stimuli 

 and to bring about the proper coordinated response of the various 

 parts of the body as in the higher animal forms. 



Endoderm. The endoderm cells, which line the enteric cavity 

 and are responsible for the nutrition of the animal, show consider- 

 able variation from the ectoderm cells. Some of them are actually 

 able to take solid food particles within the cell body in typical 

 protozoan fashion, and digest them (intracellular digestion). 

 Other endoderm cells are able to secrete digestive enzymes into 

 the large enteric cavity which digest the food so that it may be 

 absorbed (intercellular digestion) . The latter method of digestion 

 is fundamentally the same as that which takes place in the ali- 

 mentary tract of all the higher animals. 



The endoderm cells, which are specialized for intracellular diges- 

 tion, are somewhat larger than the epithelio-muscular cells of the 

 ectoderm. Projecting from many of these cells into the enteric 

 cavity are the flagella. Temporary projections of the cell, or pseu- 

 dopodia, which appear to be similar to those in Amoeba, are also 

 frequently formed. Both the flagella and pseudopodia apparently 

 aid the cells in securing food particles for intracellular digestion. 

 The internal structure of these cells varies somewhat, depending 

 upon the amount of food which is present in the cell undergoing 

 intracellular digestion. When plenty of food is present the cyto- 

 plasm contains a large number of gastric vacuoles. When food is 

 not abundant, the gastric vacuoles are reduced in number and 



