62 THE ANATOMY OF LWERTEBRATED ANIMALS. 



that tract would be a virtual nerve, although it might have 

 no optical or chemical characters which should enable us to 

 distinguish it from the rest of the protoplasm. 



It is important to have this definition of nerve clearly 

 before us in considering the question whether the lowest 

 animals possess nerves or not. Assuredly nothing of the 

 kind is discernible, by such means of investigation as we at 

 present possess, in Protozoa or Porifera ; but an}' one who 

 has attentively watched the ways of a Colpoda^ or still more 

 of a Vorticella^ will probably hesitate to deny that they 

 possess some apparatus by which external agencies give 

 rise to localized and coordinated movements. And when we 

 reflect that the essential elements of the highest nervous 

 system — the fibrils into which the axis-fibres break up — are 

 filaments of the extremest tenuit}^, devoid of any definite 

 structural or other characters, and that the nervous system 

 of animals only becomes conspicuous by the gathering to- 

 gether of these filaments into nerve-fibres and nerves, it will 

 be obvious that there are as strong morphological, as there 

 are physiological, grounds for suspecting that a nervous sys- 

 tem may exist very low down in the animal scale, and possi- 

 bly even in plants. 



The researches of Kleinenberg, which may be readily veri- 

 fied, have shown that, in the common Ilydra^ the inner ends 

 of the cells of the ectoderm are prolonged into delicate pro- 

 cesses, which are eventually continued into very fine longi- 

 tudinal filaments, forming a layer between the ectodeim and 

 the endoderm. 



Kleinenberg terms these neuro-muscular elements, and 

 thinks that they represent both nerve and muscle in their 

 undifferentiated state. But it appears to me that while the 

 assumed contractility of these fibres might account for the 

 shortening of the body of the Polyp, they can have nothing 

 to do with its lengtheninsr. As the latter movements are at 

 least as vigorous as the former, Ave are therefore obliged to 

 assume sufficient contractility in the general constituents of 

 the body to account for them. And if so, what ground is 

 there for supposing that this contractility can be exerted by 

 only one tissue when the body shortens? To my mind, it is 

 more probable that " Kleinenberg's fibres " are solely inter- 

 n uncial in function, and therefore the primary form of nerve. 

 The prolongations of the ectodermal cells have indeed a 

 strangely close resemblance to those of the cells of the olfac- 

 tory and other sense-organs in the Vertehrata y and it seems 



