204 COMPARATIVE ANATOMY 



being due to a great expansion of the two pectoral fins. The 

 commonest English rays are the thornback, Raia davata, and 

 the skate, Raia batis. The following description will apply to 

 the lesser spotted dogfish, ScvUium canicula, but except for 

 certain differences in detail, it will be found serviceable for 

 any other species of dogfish, and may even be used as a 

 guide to the dissection of skates and rays. 



The lesser spotted dogfish is a voracious and predatory fish, 

 swimming about in large shoals near the bottom of the 

 shallower seas round our coasts. It- preys upon fishes, small 

 crustaceans, octopus, and squid, and is caught in large numbers 

 by fishermen, both in nets and by hook and line, but except 

 for anatomical purposes, it has no marketable value. 



Comparing its general external features with those of 

 Amphioxus, it is at once evident that we have to do with a 

 true craniate vertebrate belonging to the class of fishes. The 

 whole length of a well-grown specimen is about two feet, and 

 the body is divisible into head, trunk and tail, though there 

 are no distinct boundaries between these regions. The head 

 is triangular in shape, somewhat flattened, and bears the three 

 pairs of organs of special sense characteristic of the higher 

 vertebrates. The olfactory organs are situated in the snout, 

 and open to the exterior by a pair of circular orifices placed 

 on the ventral surface. These nasal openings communicate 

 by wide naso-oral grooves with the mouth, the grooves being 

 partly covered and concealed by flaps of skin. The eyes, 

 situated on the sides of the head, are elongated, slit-like, and 

 furnished with movable lower eyelids. The auditory organs 

 are situated on the sides of the head immediately behind the 

 eyes, but, in the absence of any tympanic membrane or 

 external ear, they are not recognisable externally. 



The mouth is a wide crescentic opening on the ventral 

 surface of the head, about an inch behind the tip of the snout. 

 It is furnished with jaws, and each jaw is armed with several 

 rows of sharp re-curved teeth. The vent or cloacal aperture is 

 placed in the mid-ventral line, rather nearer to the anterior 

 than to the posterior end of the body. Right and left of the 

 cloacal aperture are the abdominal pores, a pair of small 

 apertures leading into the coelom, and guarded by valves. The 

 general shape of the body can be better understood from an 

 inspection of fig. 51, than from any description. It is much 



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