166 



ZOOLOGY 



SECT. 



In all recent Elasmobranchs the male has, connected with the 

 pelvic fins, a pair of grooved appendages the claspers or pterygo- 

 podia which subserve copulation. 



The mouth is situated on the ventral surface of the head, usually 

 a considerable distance from the anterior extremity. In front of 

 each angle of the mouth on the ventral surface is the opening of 

 one of the olfactory sacs, each of which is frequently connected 

 by a groove the naso-buccal groove with the mouth-cavity. 

 Behind the mouth, on the dorsal surface in the Rays, and at the 

 side in the Sharks, is the spiracle. Along the sides of the neck 

 in the Sharks, and on the ventral surface in the Rays, there is on 

 either side a row of slit-like apertures the branchial slits or branchial 



clefts. These are usually five 

 in number on each side ; but 

 in Hexanchus and Chlamydose- 

 lachus there are six, and in 

 Heptanchus seven. In Chlamy- 

 dosdachus (Fig. 841) a fold 

 comparable to a rudimentary 

 operculum . extends back over 

 the first branchial cleft, and is 

 continuous across the middle 

 line ventrally ; in the re- 

 mainder of the sub-class no 

 such structure is represented. 

 A large cloacal opening is 

 situated just in front of the 

 root of the tail, and in most 

 members of the sub-class a 

 pair of small openings placed 

 close to it the abdominal pores 

 lead into the abdominal 

 cavity. 



When the integument de- 

 velops any hard parts, as is the 

 case in the majority of the Elasmobranchs, they take the form, not of 

 regular scales, as in most other fishes, but of numerous hard bodies 

 (Fig. 844) which vary greatly in shape, are usually extremely minute, 

 but are in some cases developed, in certain parts of the surface, into 

 prominent tubercles or spines. When these hard bodies are, as is 

 commonly the case, small and set closely together in the skin, they 

 give the surface very much the character of a fine file ; and the skin 

 so beset, known as " shagreen," was formerly used for various 

 polishing purposes in the arts. This is the placoid form of exoskele- 

 ton, to which reference has been already made. Each of the hard 

 bodies has the same structure as a tooth, being composed of dentine, 

 capped with an enamel-like layer, and supported on a base of a sub- 



FIG. 843. Sting-Ray (Urolophus cruciatus). 

 (After Giinther.) 



