252 COMPARATIVE ANATOMY 



the epiderm, 1 but in other Fishes (as is also the case on the head 

 in Dipnoans) they may eventually become enclosed in depressions 

 or complete canals: these are often branched, and are formed 

 either by the epiderm only, or more usually, by the scales and 

 bones of the head, and they open externally from point to point 

 (Fig. 184, c). 



FIG. 185. SENSORY CANALS or Chimcera monstrosa. (After F. J. Cole.) The 

 innervatioii is indicated by the different kinds of shading. 



(1.) Supra-orbital canal (innervated by superficial ophthalmic of facial 



cross-hatched : the black segment is the portion innervated by the pro- 



fundus) = cranial (C) 4- rostral (/?) + sub- rostral (SJR). 

 (2.) Infra-orbital canal (buccal + otic of facial dotted) = orbital (Or] + 



sub-orbital (SO) + portion of angular (A)+ nasal (N). 

 (3.) Hyomandibular or operculo-mandibular canal (external mandibular of 



facial black ) = remainder of angular (A) + oral (0) + jugular (/). 

 (4.) Lateral canal (lateral line branch of vagus oblique shading) = lateral 



(L) + occipital (Oc) + aural (A u) + post-aural (PAu). 

 The small dots on the snout represent the apertures of ampullary tubes. 



The distribution of these sensory organs extends over the whole 

 body, but (except, e.g. in Petromyzon), they are situated character- 

 istically along certain tracts, the position of which is very constant: 

 on the head, supra-orbital, infra-orbital, and hyomandibular tracts 



1 At the time when an Amphibian undergoes metamorphosis and gives up its 

 aquatic habits, these sensory organs sink downwards into the deeper layer of the 

 skin, and, as the epiderm grows together over them, they become shut off from 

 the exterior and reduced, and may finally disappear (Anura and certain Caduci- 

 branchiata). In others of the latter group, in which they are retained and also new 

 ones are formed, they come to the surface when the animal returns to the water 

 during the breeding season (Fig. 184, A). Peculiar sense-organs are present in 

 the aquatic Xenopus and in Ichthyophis glutinosus. The horny wart-like 

 structures arising periodically during the breeding season in Cyprinoids, and 

 known as "pearl-organs," are due to a modification of the reduced nerve- 

 eminences, Similar structures occur in Anura. 



