20 ; H. B. POLLARD. 



distinction established by His, nerves of the lateral cornu. The sensory 

 nerves of the oral cirri are branches of the Trigeminus, and a sharp 

 distinction must be drawn between them and the nerves of the lateral 

 line system. The nerves of the lateral line system develope quite 

 differently from the trigeminal branches. Their ganglia are derived 

 from cells proliferating along certain tracts of the ectoderm as shown 

 by Beard, Froriep, and Kupffer. I have followed the process myself 

 in Gobius. The evidence of comparative anatomy is not less clear as 

 to distinctness of the lateral line nerves in fish. 



The topographical position and course of nerves is not; of great 

 importance. This has been determined by Stannius for the palatine 

 nerves. "It is to be established that in certain classes of animals a 

 larger, in others a smaller portion of allied elements may be contained 

 originally in the coarse of one or other of two allied nerves, and at the 

 same time the same elements may frequently arise by an indifferent 

 root, without distinctly belonging to the one or the other nerve." 



Numerous examples of this phenomenon will occur in this paper, 

 The most extreme case is that of the premaxillary nerve of Myxiuoids. 

 According to Fiirbringer the premaxillary nerve in Bdellostoma, runs 

 over the eye-stalk and optic nerve, while in Myxine (and all other 

 vertebrates) it runs below the optic nerve. Fiirbringer indeed explains 

 this by supposing the eve a later structure and capable of wandering, 

 but an explanation on the grounds given by Stannius is more reason- 

 able. Thus we see that the fundamental grounds for determining the 

 homology of nerves are (1) origin from homologous nerve cells, (2) 

 terminal distribution to definite structures. The course of the fibres 

 is of less importance. 



It is hardly necessary to remark, how T ever, that an absolutely strict 

 conception of homology is incompatible with a theory of evolution. 



The sensory nerve of the nasal tentacle is the ophthalmicus pro- 

 fundus, and it is shown best in Trichomycterus where it takes the 

 normal course of an ophthalmicus profundus, namely, below the rectus 

 superior and obliquus superior, over the eyestalk. In Trichomycterus 

 it runs on the outer side of the olfactory organ to reach the tentacle. 

 In Clarias the nerve does not bear quite the same relation to the eye 

 muscles, because the small eye, along with the muscles, is shifted very 

 far laterally, while the nerve follows the skull wall more closely. It 

 runs on the median side of the olfactory organ to reach the tentacle. 



