386 



FISHES 



the supra-orbital canal. Transverse commissural canals often 

 connect the lateral and supra-orbital canals of opposite sides 

 across the dorsal surface of the head, and the corresponding 

 infra-orbital and hyo-mandibular canals may also be continuous 

 at the extremity of the snout or at the mandibular symphysis. 



Throughout their extent the canals communicate with the 

 exterior by pores, or short canals terminating in pores, or by 

 branched canals ending in groups of pores. In Cyclostomes l the 



lateral line system 

 is represented by 

 pit-organs disposed 

 as in Fishes, and 

 innervated by a 

 true lateralis nerve. 

 Some Elasmo- 

 branchs have the 

 lateral canal of the 

 trunk represented 

 by an open groove 

 protected by mar- 

 ginal denticles. 

 Chimaera is more 

 FIG. 220. Sensory canals of the left side of the head of primitive Still ill 

 Gadus virensi e, Eye; i.o, infra-orbital canal (dotted) ; . 



l.c, lateral canal (oblique shading) ; n, nasal apertures ; this respect, lor Oil 

 op, operculum ; op.m, operculo-mandibular canal (longi- fV<e head as well 

 tudiual shading) ; p.o, pit-organs ; s.o, supra-orbital canal 



(cross - hatched) ; s.o.c, supra - orbital commissure; s.t, as Oil the body the 

 supra-temporal branch ; t.t, tubuli by which the canals cp , ianvv nro -cir,c 

 communicate with the exterior. (From Cole.) O1 -> 



in open grooves. 



Amongst Fishes these organs are most primitive in the Dipnoi, 

 where they retain their superficial position in the epidermis. In 

 Teleostomes the lateral canals perforate the scales of the lateral line, 

 and at intervals they open externally by simple or multiple pores 

 which perforate the scales. On the head they are more or less 

 completely enclosed in special ossicles which either remain distinct 

 or fuse with certain of the adjacent dermal or cartilage bones of 

 the skull. The use of the lateral line organs is not certainly 

 known. They occur only in Fishes and Amphibia, and as blind 

 Fishes are able to avoid obstacles with the greatest ease when 

 swimming, it is possible that these organs enable their possessors 

 1 Johnston, Journ. Comp. Neurology, xii. 1902, p. 2. 



