191 !•] THE PRIMITIVE NERVOUS SYSTEM. 221 



or at least with this member present in only a most primitive state. 

 In my opinion this is the condition in most coelenterates. Judging 

 from the more recent work on the nervous organs of these animals, 

 centralization can scarcely be said to be present at all in hydra ; it is 

 but little more pronounced in the sea-anemone ; and, though most 

 marked in the jellyfishes, it does not rise even here to a grade that 

 entitles it to comparison with what is seen in such forms as the 

 earthworm. The coelenterates. then, are animals possessing recep- 

 tors and effectors but without developed adjustors. Hence the 

 adjuster or central organ is in all probability an acquisition that 

 represents a later stage in the development of the neuromuscular 

 mechanism than that seen in the coelenterate. 



If the coelenterates represent a stage in the evolution of the 

 neuromuscular mechanism in which sensory cells and muscles are 

 the only important parts present, it is natural to ask if there is not a 

 still more primitive state from which the C(xlenterate condition has 

 arisen. On this question several hypotheses have already been ad- 

 vanced. Claus and, subsequently, Chun maintained that originally 

 the ner^•ous system and the muscles were differentiated indepen- 

 dently and that they became associated only secondarily. This view 

 has deservedly received very little attention, for not only is it difficult 

 to conceive that an animal would develop receptive ability without 

 at the same time acquiring the power to react, but not a single exam- 

 ple among the lower animals is known in which developed nerve and 

 muscle are present and independent of each other. 



Much more worthy of consideration than the hypothesis of the 

 independent origin of nerve and muscle is Kleinenberg's theory of 

 the neuromuscular cell. In 1872 Kleinenberg announced the dis- 

 covery in the fresh-water hydra of what he designated as neuro- 

 muscular cells. The peripheral ends of these cells were situated on 

 the exposed surface of the epithelium, of which they were a part 

 and were believed to act as nervous receptors ; the deep ends were 

 drawn out into muscular processes and served as eft'ectors to which 

 transmission was supposed to be accomplished through the bodies 

 of the cells. Each such cell was regarded as a complete and inde- 

 pendent neuromuscular mechanism, and the movements of an animal 

 provided with these cells was believed to depend upon the simul- 



