COELENTERATA 



17 



is the stalk necessary for reflex movements of the tentacles and the 

 proboscis. 



The neuro-muscular organization of Corymorplia is diffuse and 

 in no sense centralized. 



Hydroid Medusae. Although the nervous system of medusae 

 is of the difi'use type, there are concentrations of the network at 

 certain places. In Gonionemiis there is a double ring of nerve cells 

 and fibers about the margin of the bell. Hyde, '02, mentions a third 

 ill-defined ring; this might be considered to be a part of the diffuse 

 network or plexus which is found over the surface of the sub- 

 umbrella. In addition to the two main marginal bands, there are 

 concentrations of nerve cells and fibers following the four radii of 

 the bell, and the manubrium has some concentration of nerve cells 

 and fibers. 



Although the nerve ring is usually double, sometimes it is not 

 divided. The nerve tissue is between the ectoderm and the muscu- 

 lar tissue. In some forms the peripheral system is but poorly de- 

 veloped with only a few nei've cells scattered beneath the surface. 



Fig. 5. Nerve cells from various coelenterates from a number of sources. 

 A. Nerve cells from Hydra, Schneider. B. Nerve cells and sense cells 

 from jelljiish from Kasseanow. C, E. Nerve plexus from Siphono- 

 phora, Schneider. D. Sense cells and nerve cell in Hydra. F, G. Nerve 

 cells and fibers in the epithelium of Hydra, Wolff. H, I. Nerve cells 

 from actinian, Hertwig. J. Nerve cells from Cerianthus, Grosley. 

 K, L. Nerve cells from actinian, Havert. 



