PHYSIOLOGY OF THE INVERTEBRATA. 297 



fibres consists of parallel spindle-shaped cells, which are set 

 perpendicularly to the outer surface of the sponge ; they end 

 in extraordinarily fine tips. The protoplasm contains small, 

 but highly and doubly refractive, granules embedded in a 

 single refractive substance. The granules are so arranged as 

 to give the appearance of a kind of transverse striation. These 

 are muscle-fibres. 



If the investigations of Ledenfeld are correct, we have in 

 these animals the beginning of a true nervous system. 



The Ccelenterata. 



Kleinenberg has shown that in Hydra the cells of the 

 ectoderm terminate internally in delicate processes from 

 which fine longitudinal filaments are produced. These fila- 

 ments form a layer between the ectoderm and endoderm. 

 According to Kleinenberg, these filaments are the represen- 

 tatives of both muscle and nerve ; in fact, he regards them 

 as neuro-muscular elements in an undifferentiated state. 

 But Prof. Huxley believes that Kleinenberg's fibres " are 

 solely internuncial 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 olfactory and other sense-organs in the Vertebrata ; 

 and it seems probable that they are the channels by which 

 impulses affecting any of the cells of the ectoderm are con- 

 veyed to other cells and excite their contraction." 



Dr. G. J. Eomanes, F.E.S.,* has shown that in the Medusce. 

 we find phenomena similar to nervous transmission sent along 

 definite tracks, or sometimes diffused from one part of the 

 body to the other, without any histological trace of differen- 

 tiated nerve-fibre. As in the Protozoa, we have in these 

 animals the early stages of the evolution of a nervous system. 



* Philosophieal Transactions of Boyal Society, 1875, p. 269; ibid. 1877, 

 p. 659 ; ibid. 1879, p. 161 ; and his book, Jellyfish, Starfish and Sea 

 Urchins. 



