THE SPINY DOGFISH {Acanthias, syn. Squalus) or 

 THE SPOTTED DOGFISH {ScylUvm) 

 THE SKATE {Raia) 



First Day 



External Features. 



Make a drawing of the Dogfish {Acanthias, or ScylUvm) viewed from the 

 side to show the following features : — 



(a) The spindle-shaped body, ending in front in a bluntly pointed snout and 



tapering off behind into an upturned tail. 



(b) The mouth, a large transverse creseentic- shaped opening on the ventral 



surface of the head. The integument covering the j aws bears numerous 

 closely set rows of teeth (modified placoid scales).. 



(c) The nostrils, a pair of rounded openings situated ventrally in front of the 



mouth. Each nostril has a short narrow flap of skin produced from 

 its anterior end over it. [In Scyllium the nostrils are connected with 

 the mouth by naso-bticcal grooves.] 



(d) The eyes, situated one on each side of the head, above the mouth. 



(e) The spiracles, modified gill-clefts through which the water enters the 



pharynx, a pair of small rounded openings, one just behind each eye. 

 Each spiracle has a rudimentary gill, the pseudobranch. 



(/) The fi-9e visceral or gill-clefts (II. -VI.), on either side of the head, some 

 distance behind the mouth. 



(g) The median and the paired fins. The median fiins are : (i) the anterior 

 and the posterior dorsal fin, along the middle of the back, each having 

 a sharp spine in front of it ; and (ii) the caudal fin, surrounding the end 

 of the (heterocercal) tail. [In some dogfishes — e.g. Scyllium — there is a 

 median anal fin behind the anus. This fin is absent in Acanthias. The 

 dorsal fins of Scyllium have no spines.] The paired fins are : (i) the 

 pectoral fins, flat and triangular, attached to the sides of the ventral 

 surface just behind the fifth gUl-cleft ; and (ii) the pelvic fins, 

 smaller and narrower than the pectoral fins but of the same shape, 

 attached on each side of the middle line on the ventral surface in 

 front of the anus. In the male their inner edges are produced into a 

 rod-like structure, the clasper, deeply grooved along its inner surface. 



{h) The cloaca, situated between the pelvic fins ; and on each side of it 

 the abdominal pore, a minute opening leading into the body-cavity 

 (abdominal or peritoneal cavity). 



(i) The lateral line (of sense organs), a longitudinal groove extending along 

 each side of the body from the spiracle to the tail region. 



(j) The openings of the sensory jelly-tubes or ampuUary canals, on the skin 

 of the head (the snout especially). 



