THE HYDRA AS A SIMPLE METAZOAN 269 



send processes to the muscle fibrils and thus mediate between the 

 cells that receive the stimulation and those making the response. 

 Taking such a mechanism as a whole, it is better called a sensory- 

 neuromuscular system than merely a nervous system. 



Hydra, therefore, exhibits in its ectoderm the machinery of 

 what is known as a receptor-effector system (c/. p. 317), since the 

 sensory cells, or receptors, receive the stimuli and transmit impulses 

 to the nerve cells, which in turn transmit them to the muscle cells 

 which are the effectors. If the gland cells of the foot are under 

 nervous control, as may be the case although it has not been 

 demonstrated, they constitute another type of effector (Fig. 

 130 K and L). This form of nervous organization is typical of 

 coelenterates. 



Within the endoderm there are only a very few cells that can 

 be regarded as nerve cells (c/. Figs. 121 and 130 B). There is no 

 evidence that these are connected with one another, although 

 they probably have processes connecting with neighboring muscle 

 cells. There are also cells that seem to be sensory and probal^ly 

 have connections with muscle processes (Fig. 130 C). The small 

 number of such nerve and sensory cells in the endoderm and the 

 sluggish nature of the movements involving the endodennal muscu- 

 lature, alike indicate that there is no unified nervous system in the 

 endoderm comparable with that of the ectoderm. As we have seen, 

 there is neither structural nor functional evidence for a connection 

 between the ectodermal and endodermal nerve cells through the 

 supporting lamella. It is probable, however, that the sphincter 

 of endodermal muscle processes surrounding the mouth may be 

 connected with the ectodermal nervous system of this region, a 

 relationship that would account for certain coordinations of ecto- 

 dermal and endodermal muscular action observed in ingestion and 

 egestion. 



At first glance it seems that the hydra has no centralization of 

 the nervous elements, but only a diffuse distribution throughout 

 the body. More careful examination reveals the existence of ring- 

 like collections of nerve cells (c/. Figs. 128 and 130 I) and a corre- 

 sponding abundance of sensory cells in the foot and hypostome, 

 the two regions of the body that are most active. The processes 

 of the nerve cells tend to run lengthwise on the tentacles and per- 

 haps on the body, although contraction of the latter region makes 

 the exact arrangement difficult to ascertain. In some other 



