158 



ZOOLOGY 



SECT. 



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

 one of the olfactory sacs, each of which is 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, is on either side a row of slit-like aper- 

 tures- the branchial slits or branchial clefts. These are usually 

 five in number on each side ; but in Hexanclms and Chlamydosc- 

 ktchus there are six, and in Heptanclius seven. In Cklamydosclachus 

 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 represented. A 

 large cloacal opening is 

 situated just in front of the 

 root of the tail, and a pair 

 of small openings placed in 

 front of it- -the abdominal 

 pores lead into the ab- 

 dominal cavity. 



When the integument 

 develops 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 which vary greatly 

 in shape, are usually ex- 

 tremely 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," is used for 

 various polishing purposes in the arts. This is the placoid form 

 of exoskeleton, 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 enamel, and supported on a 

 bony base, representing the cement or crusta petrosa of the tooth. 



The skeleton is composed of cartilage, with, in many cases, 

 deposition of bony matter m special places notably in the jaws 

 and the vertebral column. The entire spinal column may be nearly 

 completely cartilaginous (Rcxanchus and Hcptanchus), but usually 



l"i<;. 785. Sting-Ray (Urolopliv.s cruciatus). 

 (After Giinther.) 



