108 



COLLEGE ZOOLOGY 



The nervous system 



In the coelenterate we find for the first 

 time true nerve cells such as are found in 

 all the higher animals. The nervous system 

 of the hydra consists of three general types 

 of cells which are called: (1) conducting 

 and motor nerve cells, (2) sensory cells, 

 and (3) sensory nerve cells. These are dis- 

 tributed throughout the body in such a way 

 as to form a network in the epidermis; this 

 can be demonstrated by a special stain in 

 the living animal. Although often called a 

 "nerve net," the elements of the system 

 have not been demonstrated to be continu- 

 ous. Nervous elements are present in the 



gastrodermis, but in such small numbers 

 that if a gastrodermal nerve network exists, 

 it is certainly much more diffuse than that 

 of the epidermis. It is presumed that the 

 two systems are interconnected by fibers 

 passing through the mesoglea, and certain 

 workers claim they have observed such 

 fibers. Circumstantial evidence for their ex- 

 istence is afforded by the fact that in certain 

 movements of the animal, inner and outer 

 layers of muscles work in coordination. 



Most of the conducting and motor nerve 

 cells (Fig. 53) of the hydra have several 

 processes; they conduct impulses in any di- 

 rection, and thus differ from the neurons of 



Flagella 



Food vacuole 

 Flagella 



Vacuole 



Nuclei 



Gastrodermal 

 gland cell 



Circular 

 muscle fiber 



Interstitial cells 



Figure 53. Principal cell types of the hydra. (Redrawn from a drawing by Justus F. Mueller; 

 made expressly for this text.) 



