368 THE DOGFISH LESS. 



(Scyllium canicula), the Lesser Spotted Dogfish (S. catulus\ 

 the Piked Dogfish (Acanthias vulgaris\ and the Smooth 

 Hound (Mustelus vulgaris). The following description, 

 though referring mainly to Scyllium^ will apply, in essential 

 respects, to any of these. 



The dogfish has a spindle-shaped body (Fig. 98), ending 

 in front in a bluntly-pointed snout or cut-water, and behind 

 tapering off into an upturned tail. On the ventral surface 

 of the head is the large, transversely elongated mouth (inth\ 

 supported by a pair of jaws which work in a vertical, and 

 not, like those of the crayfish, in a transverse plane, and 

 are, in fact, portions of the skull, having nothing to 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 (na\ each lead- 

 ing into a cup-like nasal sac. The eyes (e) are also two in 

 number and are placed one on each side of the head, above 

 the mouth. Behind the mouth are five slit-like apertures 

 (ex. br. ap\ arranged in a longitudinal series : these are the 

 gill-clefts or external branchial apertures. Just behind the 

 eye is a small aperture, the spiracle (sp) : like the gill-clefts, 

 it communicates with the pharynx, and it is found by de- 

 velopment to be actually the functionless first gill-cleft. 



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

 between its two ends, is the anus or cloacal aperture (an\ 

 and on either side of it a small hole, the abdominal pore, 

 opening into the ccelome. 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 longitudinal streak (/. /) on each side of the body, con- 

 nected in front with a series of branching lines on the head 

 and continued backwards to the tail, is known as the lateral 



