454 CHONDKICHTHYES [CH. 



development before attaining the adult condition. Even in the few 

 aberrant cases where fertilisation is internal and where the young 

 are produced viviparously these young are nevertheless very small 

 compared to the size of the parents. 



Sub-class I. CHONDRICHTHYES. 



v 



The Chondrichthyes include all the creatures known as Sharks, 

 Dog-fish and Rays whilst the Osteichthyes include all other fish. 

 The placoid scale, the only form of skin defence developed in this 

 sub-class is a little spikelet consisting of a substance called 

 dentine coated with enamel. The spikelet covers a little cone- 

 like projection of the dermis, and it is covered in turn by the epider- 

 mis or true ectoderm. Dentine is a hard substance produced by 

 the calcification of the ground substance of the dermal connective 

 tissue. It differs from bone in the fact that it includes no cells 

 within it but from the cells of the connective tissue in the cavity 

 which it surrounds and which is called the pulp-cavity, processes 

 are given off which penetrate the dentine and give rise to canals 

 called dentinal canals. The core of soft connective tissue is 

 called the dentinal pulp. 



The spike therefore may be described as a little wart of dermis 

 calcified on the outside. It pushes the ectoderm before it, and it 

 becomes encrusted with crystals of carbonate of lime forming the 

 enamel layer (Fig. 221). These closely set crystals are secreted by 

 the inner or basal ends of the ectoderm cells. One would naturally 

 expect that structures like scales, which are closely arranged all over 

 the body, would also invade the stomodaeum, which is merely a part 

 of the skin. This we find to be the case, but here the scales are 

 very greatly enlarged in size and changed in function ; they are the 

 well-known teeth which are used for the purpose of retaining and 

 lacerating prey which has been seized. The spike of the tooth is 

 usually flattened and blade-like, and provided with strongly serrated 

 edges. Fusions of several teeth can occur. The teeth are developed 

 in a deep fold of skin, part of the stomodaeum, situated just inside 

 the lower jaw, and usually speaking only the outermost row are in 

 use at one time, the skin working forward the next set as each row 

 wears out. 



The Chondrichthyes are divided into two orders, viz. the Elasmo- 

 branchii and the Holocephali. In the former group the visceral 

 arches are all distinct from the cranium. The gills are borne on 



