XIII 



PHYLUM CHORDATA 



171 



pelvic fins, a pair of otoo\-c(1 ap])ciidagcs — the ckiRprrs ov pterygo- 

 jpodw — which subserve copulation. 



Thi' mouth is situated on the ventral surface of the head, usually 

 a considerable distance from the anterior extrenut3\ In front of 

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

 one of the olfactory sacs, each of wliich 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 Ohlo/nydose- 

 lachus there are six, and in 

 Heptanch^iS seven. In Chlauiy- 

 doselachus (Fig. 830) a fold 

 comparable to a rudimentary 

 operculum extends back over 

 the first branchial cleft, and 

 is continuous across the 

 middle line ventrally ; in the 

 remainder of the sub-class 

 no such structure is repre- 

 sented. A large cloacal open- 

 ing is situated just in front 

 of the root of the tail, and a 

 pair of small openings placed 

 in front of it — tlie abdominal 

 pores — lead into the abdom- 

 inal 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. 833) 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 exoskeleton, to which reference has been already made. 

 Each 6f the hard bodies has the same structure as a tooth, 

 being composed of dentine, capped with an enamel-like layer, and 



Fig. 832. 



-Sting-Ray (Ut-oloplms crvd(dw). 

 (After Giiuther.) 



