222 



OSTE1CHTHYES 



orbital canals is the supraorbital canal, which passes through the 

 frontal and nasals to the snout, where it may join the infraorbital. It 

 is the superior ophthalmic branch of the facialis which innervates 

 the supraorbital canal. Lastly, the ramus hyomandibularis facialis 

 supplies a hyomandibular canal, which passes down the preopercular 

 and forwards through the dermarticular to the dentary. 



It frequently happens that, in the more specialised fish, where 

 the cranial bones and scales become thin, lose their superficial 

 layers, and sink far below the surface, the system of canals again 



po. 



oc. 



sof. 



soc. 



md: 



FIG. 196. 



Diagram of the head of Amia calva, L., showing the system of lateral-line canals and pit- 

 organs, and their nerve-supply (from Allis). b, buccal branch of facial nerve ; g, dorsal branch 

 of glossopharyngeal ; h, hyomandibular branch of facial ; l.l, lateral line of trunk ; l.r, lateral- 

 line branch of vagus ; wrf, mandibular canal ; o.c, occipital or supratemporal canal ; p, pit- 

 organs on trunk ; po, postorbital canal ; ps, pit-organs on head ; sob, suborbital canal ; *o.f, 

 supraorbital canal ; s.o.f, superior ophthalmic branch of facial ; t, temporal canal. The system 

 of distinguishing the canals is indicated on the right of the figure : 1, supraorbital, and main 

 canal of trunk ; 2, postorbital ; 3, suborbital ; 4, hyomandibular (the same system is used 

 in'Fig. 85). 



becomes free. Every stage in the process of the liberation of the 

 canals from the underlying bones may be traced in the Teleostei. 

 Special tubular bones often become separated off, and are left in 

 these fish to protect the sensory canals (Figs. 328, 456). Lastly, in 

 both the Teleostei and Dipnoi the lateral line may so far degenerate 

 that the sense-organs remain, at all events along the body, exposed 

 on the surface, and no longer sink in a tube at all. 



The chondrocranium, well developed in many Osteichthyes, is 

 much reduced in the more specialised groups, and is distinguished 

 in all by the disappearance of the cartilaginous wall separating the 

 cavity of the skull from that of the auditory capsule (Figs. 303, 358). 



