464 THE DOGFISH 



CHAP. 



(VII p. s) to its anterior wall and extends along the 

 anterior border of the auditory capsule and the posterior 

 wall of the orbit, and just beneath the skin, to fhe anterior 

 side of the hyoid arch : it thus forks over the spiracular or 

 mandibulo-hyoid cleft. 



The glossopharyngeal (IX) forks above the first gill-cleft, 

 thus giving rise to two branches, one passing down the 

 posterior side of the hyoid, and the other down the anterior 

 side of the first branchial arch. The main part of the 

 vagus extends backwards to the outer side of the lateral 

 nerve and gives off four branchial nerves (X br 1 - 4 ) forking 

 over the second to the fifth gill-clefts respectively, and is 

 then continued into the visceral nerves (X v), which supply 

 the stomach and heart. 



Sensory organs. The dogfish possesses, as we have 

 seen, a series of peculiar integumentary sense-organs 

 supplied by the nerves just described, the exact function 

 of which is not known with certainty. They are situated 

 within a number of epithelial canals, developed from the 

 epiderm, the openings of which on the head have already 

 been noticed (p. 432). 



These tubes are of two kinds, known respectively as 

 sensory and ampullary canals : the former, which are at 

 present in all Vertebrates with gills (p. 430), are all con- 

 tinuous with one another and are situated along certain 

 definite tracts on the head and jaws, a canal extending along 

 the body and tail as the lateral-line canal. The ampullary 

 canals, which are peculiar to Elasmobranch fishes, and 

 which contain a gelatinous material, are not continuous 

 with one another, but run side by side, and form large 

 masses in the snout and at the sides of the head ; at their 

 blind ends they are swollen to form ampulla, to which the 

 nerves are distributed. The sensory cells are arranged in 

 little conical masses in the lining epithelium of the sensory 

 canals and of the ampullae, a section of one of which nearly 

 resembles that of an ampulla of a semicircular canal of the 

 ear (Fig. 60). 



The olfactory organs are a pair of cup-like sacs in the 

 snout, enclosed by the olfactory capsules and opening 

 externally on the ventral side by the nostrils. Notice 

 that there are no internal nostrils, as in the frog : these 



