386 FISHES CHAP. 
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* 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 18 more 
Fic. 220,—Sensory canals of the left side of the head of primitive still in 
Gadus virens. e, Eye ; 7.0, infra-orbital canal (dotted) ; : 
l.c, lateral canal (oblique shading) ; 7, nasal apertures ; this respect, for on 
op, operculum ; op.m, operculo-mandibular canal (longi- the head as well 
tudinal shading) ; p.o, pit-organs ; s.o, supra-orbital canal : 
(cross-hatched) ; s.0.c, supra-orbital commissure; s.f, aS on the body the 
supra-temporal branch ; ¢.¢, tubuli by which the canals as ASAE. GIG 
communicate with the exterior. (From Cole.) inept organs are 
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 
ma 
? Johnston, Journ. Comp. Neurology, xii. 1902, p. 2. 
