296 PHYSIOLOGY OF THE INVERTEBRATA. 



cells of the Protozoa, are the means of conveying nervous 

 energy. If no nervous system is anatomically differentiated, 

 there is every reason to believe that the protoplasm contains 

 a " diffused nervous system" (Gruber). 



In these organisms innervation is rudimentary ; and the 

 nervous function devolves upon the protoplasm, which is the 

 cause of the phenomena of contraction, secretion, &c., and 

 according to M. Binet, of certain psychical acts. 



Certainly no definite nerve-tracts have been discovered in 

 these animals; "but any 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 localised and co- 

 ordinated 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 

 tenuity devoid of any definite stractural or other characters, 

 and that the nervous system of animals only becomes con- 

 spicuous by the gathering together 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 system may exist very low down 

 in the animal scale, and possibly even in plants." (Huxley.) 



The Porifera. 



No differentiated nervous system has been discovered, but 

 there is little doubt that nervous function is traceable in the 

 protoplasm of these animals. 



The nervous system of the horny sponges has been 

 recently examined by Dr. E. von Ledenfeld.* He gives an 

 account of Eiis^pongia anfraduosa, which differs in some 

 particulars from Eusponcjia officinalis (the bath sponge). 

 The fine membrane which extends from the tips of the horny 



* Sitzungaberichte der Kgl. Preussischen Alcadcmu. dtr Wisnenschaften in 

 Berlin, 1885, p. 1015. 



