Chap. IV.] THE NERVOUS SYSTEM OF MOLLUSKS. 



71 



the Tunicata. Their Hfe of relation with the external 

 world is of the simplest description. They are sta- 

 tionary creatures, having even no prehensile organs — 

 their food being brought to the commencement of the 

 alimentary canal by ciliary action. 



In correspondence with such a simple mode of life, we 

 might expect to find a very rudimentary 

 nervous system, and this expectation is fully 

 realized. The Tunicata possess a single 

 small nervous ganglion lying between the 

 bases of the two funnels through which 

 water is taken in and discharged (fig. 21, c). 

 This ganglion receives branches from the 

 tentacula guarding the orifice of the oral 

 funnel, and possibly from the branchial 

 chamber ; whilst it gives off outgoing fila- 

 ments to the various parts of the muscular 

 sac, and perhaps to the alimentary canal rough diagram- 



T i? ii J.1 • 1 1 T raatic sketch of 



and some ot the other internal organs. In j^g Nervous Sys- 

 some of the solitary Tunicata a rudimentary t^m. (Soiiy after 



, p ,. . , . . Cuvier.) a, Bran- 



visual function is presumed to exist. At cwai orifice ; b, 

 all events, pisjment-spots are situated on, or excretory onfice ; 



' i- CI L ' c, nerve ganghon 



in very close relation w^ith, the solitary with its afferent 



and 

 nerves. 



efferent 



ganglion. 



The recent investigations of Kupffer tend 

 to show that this extremely simple nervous system, never- 

 theless, represents a decidedly higher type of organization 

 than had been previously supposed. Further details 

 cannot, however, here be given.* 



The Brachiopods are among the oldest and most wide- 

 spread of the forms of life in the fossil state, and the 

 geographical distribution of their living representatives at 



* See Gegenbauer's " Comp. Anatomy," English Translation, 

 p. 395, 



