SENSE-ORGANS 



385 



subdivision of the primary pores into groups of surface-pores, 

 complete the evolution of the system in its more advanced 

 condition. Typically, the lateral line system consists of certain 

 canals or grooves, usually but not invariably continuous, and 

 denned by their innervation, (i). a lateral canal extending along 

 the side of the body and the hinder part of the head, and having 

 its .sensory organs supplied by the great lateralis nerve (Fig. 

 220); (ii.) a supra-orbital canal passing forwards over the eye 

 and innervated by the superficial ophthalmic branch of the 

 facial nerve ; (iii.) an infra-orbital canal supplied by the buccalis 



FIG. 219. Vertical longitudinal section through the lateral canal of Amia calra. l.n, 

 Lateralis nerve with its branches, n, n, to the sensory organs, s.o, s.o ; p,p,p, external 

 pores ; s.c, sensory canal ; s, s, scales of the lateral line. (From Wiedersheim, after 

 Allis.) 



and otic branches of the same nerve ; and (iv.) a hyo-mandibular 

 or operculo-mandibular canal, situated on the outer side of the 

 hyoid region, and thence prolonged downward and forward in 

 relation with the lower jaw, and innervated by the external 

 mandibular branches of the facial nerve. The hyo-mandi- 

 bular canal is sometimes distinct from the other canals, as in 

 Elasmobranchs and some Teleosts (Fig. 220); and in certain 

 North American Siluroids the same may be said of the supra- 

 orbital. But, as a rule, the infra-orbital is continuous behind 

 both with the lateral and the supra-orbital canals, while the 

 hyo-mandibular canal joins the infra-orbital, or, exceptionally, 

 VOL. VII 2 C 



