236 FISHES 



Quite distinct from this cartilaginous endoskeleton are the 

 calcified structures in the skin which form the external skeleton, 

 or as it is technically termed exoskeleton. In the dog-fish these 

 consist of small tooth-like structures which are called dermal 

 denticles, that is to say skin-teeth. The skin of the fish, as of 

 other vertebrates, consists of two layers, an outer cellular layer 

 without blood-vessels, the epidermis, and a thicker internal 

 layer, the derma, which is fibrous and supplied with blood- 

 vessels. The dermal denticles are situated in the derma ; 

 each consists of a flat base having the structure of bone and a 

 pointed spine projecting backwards and outwards. The ex- 

 ternal layer of the spine consists of enamel which is secreted at 

 the inner surface of the epidermis ; below it is a layer of dentine 

 or ivory, the characteristic substance of teeth, and within this 

 is the tooth cavity containing vascular pulp and opening by an 

 aperture in the basal plate. These denticles have the same 

 structure as the teeth in the jaws, and the teeth of other verte- 

 brates are essentially similar. On the surface of the body in 

 the dog-fish the denticles are everywhere arranged in regular 

 oblique rows ; in other cases they may be much larger in certain 

 regions and absent in others, as for example in the skates and 

 rays. In other fishes the points of the teeth disappear every- 

 where except in the jaws, and the teeth themselves are modified 

 into flat bony plates in the skin, covered on the outer surface with 

 a smooth layer of substance resembling dentine, and called 

 ganoin. These ganoid scales may form a complete armour over 

 the body and head, as in the Bony Pike (Lepidosteus) of North 

 America (Plate XX., C) ; such fishes are often called ganoid 

 fishes. Such plates over the head and pectoral girdle come into 

 intimate connexion with the internal skeleton, which in bony 

 fishes is ossified also into distinct bones. In the most specialised 

 fishes like the gold-fish, which are called Teleostei, the dermal 

 plates lose their outer layer of enamel and on the head and pectoral 

 girdle form the superficial bones of the skull and fin-girdle, while 

 on the body they are modified into the thin overlapping scales, 

 which may be either smooth-edged or cycloid as in Fig. 20, A, 

 or may have the exposed posterior border furnished with small 

 spines as in Fig. 20, B. Thus in the bony fish the skeleton 

 consists of the ossified internal skeleton united in the head and 

 pectoral girdle with bones derived from the external skeleton 



