4i6 THE DOGFISH chap. 



do with limbs. They are covered with 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 placed one on each side of the head, above the 

 mouth : they are protected by folds of the skin form- 

 ing upper and lower eyelids, the latter of which can be 

 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. 403). 

 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 opens a minute hole, the abdominal pore, commu- 

 nicating with the coelome, which is therefore not a com- 

 pletely closed cavity, as in the frog. From the end of the 

 snout to the last gill-cleft is considered as the head of the 

 fish ; 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- 



