154 THE SPINY DOGFISH -THE SKATE 



Note the placoid scales (dermal denticles or sldn-teeth) covering the whole 

 body, also two unroofed areas of the skull, the fontanelles, which can be felt 

 through the integument of the head. 



Fig. 51.— The Spiny Dogfish {Acanthiae, ayn. Squcdwi). Reduced. 



[Make a drawing of the Skate {Raia) viewed from above to show the following 

 featu/res : — 



(a) The rhamboidal-shaped body, flattened dor so-ventr ally, ending in front in a 



pointed snout and produced posteriorly into a whip-like (heterocercal) tail. 



(b) The eyes, situated near the anterior end. 



(c) The spiracles, a pair of rounded apertures situated juM behind the eyes, each 



containing a rudimentary gill (the pseudobranch). 



(d) The large pectoral fins forming the sides of the body. 



(e) The smaller pelvic fins behind the pectoral fins, one on each side of the base 



of the tail. In the male, claspers are present as in the Dogfish. 



(f) The small unpaired fins and the spines on the tail. 



(g) The small caudal fin surrounding the end of the tail. 



Note the placoid scales on the skin {numerous in certain species of Skate), 

 also the fontanelles, the unroofed areas of the skull. 



Make a drawing of the Skate viewed from below to show the follomng features : — 



(a) The mouth, a large transverse opening. The integument covering the jaws 



bears numerdu^ closely set raws of teeth. 



(b) The. nostrils, situMed in front of the mouth, and connected each by a naso- 



bu£cal groove with the angles of the mouth. 



(c) The five gill-clefts on each side of the body, behind the mouth. 



(d) The cloaca situated between the pelvic fins, and, on each side of it, the 



abdominal pore opening into the body-cavity, 

 {e) The sensory jelly-tubes of the skin.] 



Internal Structure. 



Remove a piece of the skin from the side of the body (towards the tail) to 

 show the myotomes. Cut off the tail region of the body (behind the cloaca) 

 and make a sketch of the cut surface to show the skin, muscles, centrum of 

 vertebra with remains of the notochord, dorsal neural arch enclosing the spinal 

 cord, and ventral hsemal arch enclosing the caudal artery (dorsal aorta) and the 

 caudal vein (below the artery). 



