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The Skate (Raia) and the Dogfish (Scyllium). 

 Types of the Order Elasmobranchii. 



Mention the external Features of the Skate and the Dogfish. 



^) The Skate, flattened dorso-ventrally, is rhomboidal in 

 shape. The anterior pointed end is the snout ; the posterior 

 end is produced as a whip-like tail (heterocercal), bearing 

 the small upaired fins. The broad triangular sides are the 

 expansive pectoral fins and (behind these) the smaller 

 pelvic fins, which (in the male) have modified portions, the 

 copulatory claspers. The dorsal surface of the body is 

 pigmented, and, in some species, is studded with dermal 

 denticles or skin teeth (placoid scales. See teeth, page 195). 

 Two unroofed areas (fontanelles) of the skull can be felt 

 through the integument covering the head. Behind the 

 eyes are the first gill-clefts, the spiracles, which open in- 

 ternally into the buccal cavity. Each spiracle has a rudi- 

 mentary gill (pseudobranch). 



On the pale or white undersurface of the body, and 

 widely apart, are the ventral nostrils, connected by naso- 

 buccal grooves with the angles of the transverse ventral 

 mouth. The folds of skin covering the jaws have closely- 

 set rows of teeth (modified placoid scales). At a width 

 apart greater than that of the mouth, and behind it, are 

 the two rows of gill-clefts (five pairs). At the base of the 

 tail is the cloaca, and beside it there are two abdominal 

 pores ; these are the external openings of the abdominal 

 or peritoneal cavity, which contains the viscera, and which 

 is roughly indicated from the outside as that large area 

 between the conspicuous pectoral girdle in front and the 

 pelvic girdle behind. A small area immediately in front 

 of the pectoral girdle indicates the pericardial portion 

 of the coelome. The skin has numerous sensory jelly 

 tubes, and glandular colls from which slime is freely 

 exuded. 



The Dogfish is elongated, somewhat flattened anteriorly, 

 ateraliy compressed behind and tapering. Behind the 

 head are the pectoral fins, freely projecting from the sides 

 of the body ventrally ; at some distance behind are the 

 smaller pelvic fins, also ventral but close together. There 

 are two dorsal fins, a ventral anal fin, and a caudal fin (tail 

 heterocercal). Along each side of head and body there is 

 a groove, the lateral line. The skin is rough (shagreen), 

 being thickly coated with small placoid scales. The other 

 external features correspond with those of the Skate, 



