NERVOUS SYSTEM. 405 



work took its origin from processes of the sense-cells. The processes of the 

 different cells probably first met and then fused together, and, becoming 

 more arborescent, finally gave rise to a complicated network. 



The primitive relations between the 

 nervous network and the muscular system 

 are matters of pure speculation. The 

 primitive muscular cells consist of epithe- 

 lial cells with muscular processes (fig. 238), 

 but the branches of the nervous network 

 have not been traced into connection with FIG. 238. MYO-EPITHELIAL 



the muscles in any Ccelenterata except CELLS OF HYDRA. (From Gegen- 

 the Ctenophora. In the higher types a baur 5 after Kleinenberg.) 

 continuity between nerves and muscles ' contractile fibres; processes 



in the form of motorial end plates has 



been widely observed. Even in the case of the Ccelenterata it is quite 

 clear from Romanes' experiments that stimuli received by the nerves are 

 capable of being transmitted to the muscles, and that there must therefore 

 be some connection between nerves and muscles. How did this connection 

 originate? 



Epithelial cells with muscular processes (fig. 238) were discovered by 

 Kleinenberg (No. 324) in Hydra before epithelial cells with nervous pro- 

 cesses were known, and Kleinenberg pointed out that Hydra shewed the 

 possibility of nervous and muscular tissues existing without a central nervous 

 system, and suggested that the epithelial part of the myo-epithelial cells was 

 a sense-organ, and that the connecting part between this and the contractile 

 processes was a rudimentary nerve. He further supposed that in the subse- 

 quent evolution of these elements the epithelial part of the cell became a 

 ganglion-cell, while the part connecting this with the muscular tail became 

 prolonged so as to form a true nerve. The discovery of neuro-epithelial 

 cells existing side by side with myo-epithelial cells demonstrates that this 

 theory must in part be abandoned, and that some other explanation must be 

 given of the continuity between nerves and muscles. The hypothetical 

 explanation which most obviously suggests itself is that of fusion. 



It seems quite possible that many of the epithelial cells of the epidermis 

 and walls of the alimentary tract were originally provided with processes, 

 the protoplasm of which, like that of the Protozoa, carried on the functions 

 of nerves and muscles at the same time, and that these processes united 

 amongst themselves into a network. Such cells would be very similar to 

 Kleinenberg's neuro-muscular cells. By a subsequent differentiation some 

 of the cells forming this network may have become specially contractile, the 

 epithelial parts of the cells ceasing to have a nervous function, and other 

 cells may have lost their contractility and become solely nervous. In this way 

 we should get neuro-epithelial cells and myo-epithelial cells both differen- 

 tiated from the primitive network, and the connection between the two would 

 also be explained. This hypothesis fits in moreover very well with the 

 condition of the neuro-muscular system as we find it in the Coelenterata. 



