NERVOUS SYSTEM, SENSES, AND SENSE ORGANS 201 



and communicating with the exterior either by a comparatively 

 small number of large pores or by numerous tiny apertures. 

 In this cephaHc system three main canals may be recognised: 

 one running forward across the hinder part of the head, bending 

 downwards below the eye and continuing on to the snout and 

 upper jaw; another running forward above the eye and also 

 ending on the snout region; and a third passing downwards 

 across the front part of the gill-cover and forwards to the lower 

 jaw. In certain regions these canals may be dilated or con- 

 stricted, or they may be variously branched. The contained 

 sense organs do not exhibit the regular arrangement character- 

 istic of those in the trunk region, and are served by branches 

 from the seventh cranial nerve. Often one or more of the 



^^.^^^^ 



Fig. 81. 

 A. Melamphaes beann,x h ; b. Kentucky Blind-fish {Amblyopsis spelaeus),x ^. 



canals becomes enclosed in a tube-like bone for the whole or a 

 part of its length ; these bones, which in the young fish develop 

 round the canal, may remain separate or become fused with 

 neighbouring bones. In certain fishes, and especially in some 

 living at great depths in the sea, the mucous canals are so much 

 enlarged in certain regions of the head that the surrounding 

 bones are excessively thin and paper-like, the whole skull being 

 soft and spongy to the touch (Fig. 81 a). 



An important point about the lateral line system as a whole 

 is its relation to the auditory organ, from which all the main 

 channels radiate. A study of the development of the inner ear 

 shows that this must have been at one time one of the sense 

 organs of the lateral line, before becoming specially enlarged 

 and modified in order to adapt it to the perception of delicate 

 sound vibrations, and to the maintenance of equiUbrium. 



