PISCES 157 



called claspers, and are used for conveying the sperms of the 

 male into the oviduct of the female. The male possesses 

 frequently a patch of spines on the dorsal aspect of the pectoral 

 fin. A further distinction is that in the male the teeth are 

 sharp and pointed, while in the female they are more flattened. 

 The skin or integument consists of a many-layered epi- 

 dermis or ectoderm based on a basement membrane and sup- 

 ported by a connective-tissue layer or dermis, and though 

 thin it is very tough. Imbedded in the dermis are the bases of 

 the sharp -pointed scales. These originate as plugs of dermal 

 cells which indent the epidermis. The dermal cells then enter 



^Epidermis 

 Dermis 



FIG. 76. Raia. Placoid scale in longitudinal section. 



upon a process which is exactly similar to tooth formation. 

 The cells next the ectoderm are called odontoblasts, and lay 

 down next the ectoderm a matrix in which lime is deposited, 

 thus forming a hard and resistent layer of dentine in the form 

 of a cup ending apically in a sharp point. This is succeeded 

 by the formation of further layers internally to the first, and 

 thus the inner or pulp cavity is gradually narrowed. The 

 scale is thickened internally, and extended below at the margin 

 of the cup. As it grows it is pushed through the epidermis 

 and forms the small or the large plaCoid scales so characteristic 

 of the Elasmobranch fishes. The pulp cavity in the process 

 is reduced to a narrow canal from which branching canals 

 radiate outwards through the dentine. Below, the pulp 

 cavity maintains a wide expansion, and the dentine is laid 

 down even more like bone in the formation of the basal plate. 

 Placoid scales are prominent usually on the dorsal surface, 



