128 THE INVERTEBRATA 



a single cell we have the receptor and effector organs which are 

 necessary for a very remarkable reflex action. Lastly, the nematocyst 

 may be attached to the base of the thread cell by the lasso, an organ 

 which helps to restrain the force of the explosion. In Hydra there are 

 no other sense organs in the ectoderm beside the cnidocils of the 

 thread cells. From the high degree of differentiation and the in- 

 dependence of action these cells might almost be considered as 

 separate organisms within the coelenterate if their development was 

 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 

 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. io8) with very small cell bodies and many 

 fine branches which anastomose into each other and also connect with 

 the sense cells in the ectoderm and endoderm. A sense cell is shown 

 in Fig. io6. It has a rod-like process projecting from the surface 

 and at its other end it ends in slender branches which join with those 

 of the nerve cells. Such sense cells respond to touch and probably 

 also to light and chemical stimuli. If a polyp is touched with a wire 

 the disturbance is transmitted in all directions by the nerve net and 

 results in a general contraction of the muscular system, which may 

 last for long periods. In some cases coelenterate polyps are only 

 capable of expansion in the absence of light. 



This ''nerve net " is the most primitive type of nervous system. The 

 cells which compose it differ from the nerve cells of higher Metazoa 

 in their simple structure, in the fact that the processes of one cell are 

 continuous into those of the cells which surround it,^ and above all in 

 the fact that they are arranged in a diffuse fashion, and not aggregated 

 along particular lines. This is at any rate true for the more primitive 

 polyps : in the medusae and the more differentiated polyps the nerve 

 system tends to concentrate in special parts but not in such a fashion 

 as to form any kind of a central nervous system. In the higher Metazoa 

 such a concentration has taken place, and with the exception of the 

 echinoderms, a nerve net only occurs in certain organs like the gut. 



Much of the interest of the coelenterates lies in the conflict between 

 the two modes of life, an easy sedentary existence and a wandering 

 or rather freely-drifting life which demands a larger measure of 

 activity and a greater elaboration of structure and physiological 

 development. The two types of individual which correspond to these 

 modes of life are the Polyp and the Medusa. There are large divisions 

 of the coelenterates in which only one type is present, while in the 



^ Synaptic junctions such as occur elsewhere in the Metazoa have, how- 

 ever, been recently demonstrated in the Scyphomedusae. 



