Chap, i.] THE NERVOUS SYSTEM IN THE ZOOLOGICAL SERIES. 36S 



nei-vous systems, more complex and more perfect, whicli we are 

 about to pass under reYiew, for there are, even in this rudimen- 

 tary network, conducting cords composed of fibres, and receptive 

 and excitation centres composed of cells. The existence of a 

 nervous system is problematical in certain radiata. A nervous 

 system seems, however, to exist in the holothuria, namely, an 

 (Bsophagian, fibrous collar, emitting radii 

 at points of which there are ganglionary 

 enlargements. 



M. D. Quatrefages found, in the 

 planaries, a nervous ganglion, situated 

 on a median Une, and formed of two 

 small masses joined together. 



The rotifera have also a central 

 ganglionary mass, situated on the 

 pharynx. This mass, sometimes divided 

 into two portions, emits nervous fila- 

 ments. 



In the colonies of bryozoaries, every 

 colonist is provided with a cerebroidal 

 ganglion, and besides, as Fritz Miiller 

 has shown, there is a system of cords 

 connecting all the individual ganglions, 

 that is to say, a colonial nervous 

 system. 



The plathelminths have, in this anterior 

 part of the body, two nervous masses, re- 

 latively large, and united by a nervous 

 commissure. The position of these ganglions has made them be 

 called cerebral. The trompe passes as if into a ring, between the 

 two cords of the commissure. From these ganglions go forth 

 two longitudinal fibrous trunks which follow , the lateral edges 

 of the body, and are provided with small ganglionary enlarge- 

 ments emitting nervous filaments (Fig. 55). 



This nervous system, so simple, is the rough model of that of 



FlQ. 63. 



Anterior extrftmity of the body of 

 a HEBmertian {Borlns iacamiUa) : 

 a, opening of the trompe ; -p, 

 trompe ;w;, vibratile fossettes; 

 n, cervical ganglion ; u', lateral 

 nervous system ; I, lateral san- 

 guineous trunks, which bend 

 forward in order to join ; before 

 this junction they send round 

 the brainabranch which unites 

 with that opposite to form the 

 dorsal vessel (a). 



