386 MANUAL OF ZOOLOGY SECT. 



spiracle. A row of slit-like apertures the branchial slits or 

 branchial clefts is present along the sides of the neck in 

 the Sharks, and on the ventral surface in the Rays. These 

 are usually five in number on each side ; but in Hexanchus 

 and Chlamydoselachus there are six, and in Heptanchus 

 seven. 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 abdominal 

 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 and are 

 usually extremely minute, but are in some cases developed 

 into prominent tubercles or spines in certain parts of the 

 surface. When these hard bodies are small and set closely 

 together in the skin, as is commonly the case, 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 what is termed 

 the placoid form of exoskeleton. 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 dermal fin-rays are horny. 



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

 deposition of calcareous matter in special places notably in 

 the jaws and the vertebral column. The entire spinal 

 column may be nearly completely cartilaginous (ffexanchus 

 and Heptanchus\ but usually the centra are strengthened by 

 radiating or concentric lamellae of calcified cartilage or 

 bone ; or they may be completely calcified. They are 

 deeply amphiccelous, the remains of the notochord per- 



