432 THE DOGFISH CHAP. 



those of the frog, portions of the skull, having nothing to 

 do with limbs. They are covered with numerous rows of 

 teeth which vary in form in the. different species. In front 

 of the mouth, on the ventral surface of the snout, are the 

 paired nostrils, each leading into a cup-like nasal or olfactory^ 

 sac, and (in Scyllium) connected with the mouth by a 

 groove. The eyes are situated one on either side of the 

 head, above the mouth : they are protected by thick folds of 

 the skin forming upper and lower eyelids, which can be 

 partially closed over the eye. Behind the mouth are five 

 pairs of slit-like apertures arranged in a longitudinal series : 

 these are the gill-clefts or external branchial apertures 

 (p. 418). Just behind each eye is a small aperture, the 

 spiracle : like the gill-clefts, it communicates with the 

 pharynx, and it is found by development to be actually the 

 functionless first gill-cleft. 



On the ventral surface of the body, about half-way 

 between its two ends, is the vent or anus, leading into the 

 cloaca (p. 23), and on either side of it a small pouch into 

 which usually opens a minute hole, the abdominal pore (Fig. 

 127) communicating with the coelome, which is therefore 

 not a completely closed cavity, as in the frog. The region 

 from the end of the snout to the last gill-cleft is considered 

 as the head of the fish; that from the last gill-cleft to the 

 ,anus as the trunk and the rest as the tail. 



A number of symmetrically arranged, minute apertures on 

 the skin of the head, particularly numerous on the snout, 

 lead into a series of tubes known as sensory canals, which 

 are situated beneath the skin in this region ; and a single 

 tube, known as the lateral-line canal^ the position of which 

 is indicated by a very faint longitudinal line, extends along 

 either side of the body and tail. The whole apparatus 

 constitutes an important, but imperfectly understood, in- 



