xii PHYLUM CHORDATA 389 



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 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 ven- 

 tral surface in the rays, 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 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, 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," 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 sup- 

 ported 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 bony matter in special places, notably in the 

 jaws and the vertebral column. The entire spinal column 



