334 EVOLUTION OF THE NERVOUS SYSTEM. 



epidenuic cells became united together so as to form a network of nerve- 

 tibres, placing the various parts of the body in nervous communication. 



(2) The process by which nerves became connected with muscles, so 

 that a stimulus received by a nerve-cell could be communicated to and 

 cause a contraction in a muscle. 



It is probable, as stated in the above summary, that the nervous net- 

 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 epithelial cells with muscular processes (fig. 238), but the 



branches of the nervous network have not 

 been traced into connection with the mus- 

 cles in any Coelenterata except the Cteno- 

 phora. In the higher types a continuity 

 between nerves and muscles in the form of 

 motorial end plates has been widely ob- 

 FiG, 238. Myo-epithklial served. Even in the case of the Coelen- 

 CELL8 OF Hydea. (From Gegen- terata it is quite clear from Romanes' 

 baur; after Kleinenberg.) experiments that stimuli received by the 



m. contractile fibres; processes 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 1 



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

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

 cesses wei'e 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 subsequent 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 explana- 

 tion must be given of the continuity between nerves and muscles. The hypo- 

 thetical explanation which most obviously suggests itself is that of fusion. 

 It seems quite possible that many of the epithelial cells of the epi- 

 dermis 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 pro- 

 cesses united amongst themselves into a network. Such cells would be 

 very similar to Kleinenberg's neuro-muscular cells. By a subsequent dif- 

 ferentiation some of the cells forming this netwoi'k may have become 

 specially contractile, the epithelial parts of tlie 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 differentiated from the primitive network, 

 and the connection between the two would also be explained. This hypo- 



