448 THE DOGFISH chap. 



skin, to the 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. '"*) forking 

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

 tinued 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 function of which is not 

 known with certainty. They are situated within a number 

 of epitheHal canals, developed from the epiderm, the open- 

 ings of which on the head have already been noticed 

 (p. 410). 



The tubes are of two kinds, known respectively as sensory and am- 

 iullary canals : the former, which are present in all Vertebrates with 

 gills (p. 414), are all continuous with one another and are situated along 

 certain definite regions in 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, converging to 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 canals or 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 are only 

 present in Vertebrates which possess lungs. The sacs are 

 lined by the olfactory epithelium, which is supplied by the 



