164 NERVE COMPONENTS OF BONY FISHES. 



trematic nerve (r. hyomandibularis) after the occlusion of 

 the spiracular canal. 



The r. mandibularis internus VII of the Ichthyopsida, 

 then, probably does not correspond to the chorda tympani 

 of the higher forms, but it is a primary component of the 

 post-spiracular nerve of vertebrates in general. When 

 absent from that nerve (as sometimes occurs) this fact, 

 rather than its presence, is to be explained as a secondary 

 modification. 



The relations of the pre-spiracular communis nerves are 

 exceedingly diverse in different vertebrates. The pri- 

 mary arrangement was doubtless the typical palatine and 

 pre-trematic branches, but even in the selachians there is 

 considerable deviation from the type in various directions. 

 As differentiation progressed, the development of taste 

 buds on the lips (correlated with the absence of communis 

 fibres in the trigeminus) led not only to the extension of 

 the typical facial nerves (palatine, pre-trematic and post- 

 trematic) from their proper arches to supply them, but in 

 some cases to the formation of entirely new nerves, such 

 as the internal rostral nerve of Acipenser, the communis 

 component of the r. maxillaris of other fishes and, as we 

 shall see below, the chorda tympani. The later develop- 

 ment of a fleshy tongue has led in higher vertebrates to a 

 similar prolongation of one or more branchial nerves to 

 innervate it. 



Now, the known diversity, even in rather closely re- 

 lated fishes, in the nerves which are thus prolonged should 

 incline to the greatest caution in establishing homologies, 

 especially those based upon peripheral relations. Thus, 

 any of four distinct communis nerves may run forward 

 upon the hyoid and mandibular arches : — (i) the r. man- 

 dibularis internus VII, (2) the r. pre-trematicus VII, 



