284 CHARACTERS OF VERTEBRATE ANIMALS 



buted section of marine eels. It attains the size of the Conger 

 and was well known to the ancient Romans, who not only 

 esteemed it as a delicacy but kept it as a pet. The skin is 

 scaleless, as in the Conger, and is brilliantly coloured. The 

 front end of the body is very thick, and the large mouth is 

 armed with powerful teeth. Pectoral fins are entirely absent. 



SUB-CLASS III. SHARKS AND RAYS (ELASMOBRANCHII) 



The Spotted Dog- Fish (Scy Ilium canwila) is a good type of 

 this sub-class, and the description already given of it (pp. 257-264) 

 will serve to give an idea of the essential features in the structure 

 of the group. The most important of those features are the 

 following: The unsymmetrical tail, and position of the mouth 

 and nasal openings on the under surface of the head. The 

 possession of spiracular clefts and at least five pairs of gill- 

 pouches, the external openings of which are not protected by a 

 gill-cover. A cartilaginous skeleton, with comparatively simple 

 skull and well-developed visceral skeleton, and paired-fin skeleton 

 on the Dog- Fish type. Numerous rows of teeth on the margins 

 of the jaws, constantly being renewed during life. A spiral valve 

 in the intestine, and a cloaca. Well-developed arterial cone in 

 the heart. A thick skin with placoid scales. No swim-bladder, 

 or at most a small tubular outgrowth from the upper side of the 

 gullet to represent it. Eggs large, containing much food-yolk. 



It is convenient to divide these fishes into two orders, one 

 containing the Sharks and Dog-Fishes (Selachoidei), the other 

 Skates and Rays (Batoidei). 



Order i. SHARKS AND DOG-FISHES (Selachoidei) (fig. 166) 



The shark-like fishes of this order comprise about 1 50 species, 

 distinguished by spindle-shaped bodies gradually tapering to the 

 tail-end, which is sharply bent up. The gill-slits open on the 

 side of the body, and the eyes possess lids. There are nine 

 families, of which only seven need be mentioned. 



i. Blue Sharks. The type of this family is the Blue Shark 

 (Carcharias glaucus), which often reaches the length of 15 feet. 

 Though not uncommon in British seas during the warmer part 

 of the year they are specially abundant in the tropics, like most 



