432 THE DOGFISH CHAP. 



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 communi- 

 cates with the pharynx, and it is found by development 

 to be actually the functionless first gill-cleft. 



On the ventral surface, about half-way between the 

 two'ends of the fish, 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 ccelome, 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 vent 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 vefy faint longitudinal line, 

 extends along either side of the body and tail. The 

 whole apparatus constitutes an important, but im- 

 perfectly understood, integumentary sense-organ : as 



