The oral cirri of Silaroids and the origin of the head !n Vertebrates 397 



rings of the nasal tube of Myxine. This view is shown below to 

 be untenable. Only the most anterior crescentic cartilage (/'of Müller) 

 corresponds to the nasal labial of Selachii and the nasal rings of Myxine. 



Nerve Supply. 



The motor nerves, which have occasionally to be referred to in 

 this paper as supplying the system of tentacular muscles, are, to 

 follow the 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 ecto- 

 derm 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 con- 

 tained originally in the course 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 Myxi- 

 noids. According to Fürbringer the premaxillary nerve in Bdello- 

 stoma runs over the eye-stalk and optic nerve, while in Myxine 

 (and all other vertebrates) it runs below the optic nerve. Fürbringer 

 indeed explains this by supposing the eye a later structure and cap- 

 able of wandering, but an explanation on the grounds given by Stan- 

 nius is more reasonable. 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 however 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 



Z««L Jthrb. VUl. Abth. I. Morph. Qg 



