COELENTERATA 



149 



of the thread cell by the lasso, an organ which helps to restrain the 

 force of the explosion. From the high degree of differentiation and 

 the independence of action these cells might almost be considered 

 as separate organisms within the coelenterate if their development 

 were not to be traced from the interstitial cells. 



The nervous system of coelenterates is one of their most character- 

 istic organs, composed of cells of a special type which are only to be 



Fig. no. Diagram of Hydra to show the nervous net. n.c. nerve cells. 



demonstrated by difficult methods of staining. Over the surface of 

 the mesogloea on both sides among the muscle tails there is spread a 

 network of cells (Fig. no) with very small cell bodies and many fine 

 branches which appear to anastomose with each other and also con- 

 nect with the sense cells in the ectoderm and endoderm. Synaptic 

 junctions such as occur elsewhere in the Metazoa have, however, 

 been recently demonstrated in the Scyphomedusae and so possibly 



