CRANIAL NERVES AND SENSE ORGANS OF FISHES. 245 



examine intermediate types, we find every gradation 

 between these two extremes — most of them amongst the 

 archaic Elasmobranchs. The first of the lateral line nerves 

 to lose individuality is the external mandibular, the buccal 

 follows, then the superficial ophthalmic, and finally the 

 lateralis. (This sequence is, of course, somewhat arbitrary, 

 but it possibly represents what actually happened.) Simul- 

 taneously with this concentration of the lateral line nerves 

 is to be noticed a concentration and diminution in the 

 lateral sense organs themselves. Primitively, the lateral 

 line nerves are not in any sense serial, "nor do they, 

 conform with any formal scheme ; latterly they are even 

 less so, and their concentration simply shadows, and is an 

 expression of, the decline and fall of the structures which 

 they supply. The whole of the facts go to show that the 

 position and course of the cranial nerves has been 

 determined by physiological necessities, and that their 

 arrangement into any formal scheme is, at the present, 

 purely artificial. 



The series of lateral sense organs, enclosed in the 

 "mucous" canals, forms an apparatus the tendency of 

 which in the specialised fish is to be reduced or even 

 lost. It is found in its most fantastic and prolific form in 

 the cartilaginous fishes, and thence upwards it becomes, by 

 an elimination of the last added details, naturally resolved 

 first into the fundamental type, which then itself begins 

 to disappear. In the various stages of their decline, 

 the sensory canals pass through some curious vicissitudes. 

 The sense organs in the canals are lost first, and then the 

 canals, being apparently functionless, often exhibit some 

 remarkable distortions. The dermal tubules become 

 reduced in number, lose their openings on to the surface, 

 and often become vesicular. The canals themselves often 

 become a.mpulliform or vesicular, and may, in apparently 



