86 



A HISTORY OF FISHES 







.-«i 



pro\ed durability of shark-leather is largely due to the rein- 

 forcement provided by these structures (cf. p. 401). 



In embryonic development the denticles make their appear- 

 ance as minute conical or papilliform outgrowths of the skin; 

 the outer epidermal layer of each of these cones becomes 

 transformed into a coat of hard enamel by the deposition of 

 lime salts; the dermal portion gives rise to dentine or ivory— 

 the characteristic substance of teeth — with a central pulp 

 cavity containing the blood-vessels and nerve (Fig. 37A). The 

 base of the cone spreads out to form the bony plate, in the 



centre of which is a hole for 

 the passage of the blood- 

 vessels and ner\'e. The im- 

 portant point to notice about 

 these denticles is that, both 

 in their structure and in the 

 manner of their development, 

 they are strictly comparable 

 to the teeth, which have 

 quite clearly been derived 

 from them {cf. p. 120). 



The arrangement of the 

 denticles already described 

 is that found in the familiar 

 Dog - fishes [Scyliorhinus, 

 Squalus, Mustelus)^ as well as 

 in most other Sharks, but 

 the denticles themselves pre- 

 sent considerable differences 

 in form and size, being some- 

 times flat, sometimes spine- 

 like, and sometimes taking the 

 form of rounded knobs. In the Bramble Shark {Echinorhinus) , 

 however, they are distributed irregularly over the body, and 

 appear as large rounded tubercles of varying size, each sur- 

 mounted by a tuft of fine spines (Fig. 36c). In the Rays {Raia) 

 they are generally scattered sparsely and unevenly over the 

 upper surface of the disc formed by the head, body, and pec- 

 toral fins; they are usually most prominent along the middle 

 line of the back and on the upper part of the tail, and may be 

 sharply pointed, flattened, or reduced to mere knobs (Figs. 

 14B; 107). In the Thornback Ray (Raia clavata) the greatly 

 enlarged denticles are known as "bucklers" (Fig. 36d), and in 





?^^^: 



B 



Fig. 37. — DEVELOPMENT OF DENTICLES 

 AND SCALES. 



A. Vertical section through the skin of 

 an embryo shark. (After Gegenbaur); 



B. Diagrammatic longitudinal section 

 through the skin of a Bony Fish, to show 

 position of scales. (After Boas.) 



d.y dermis ; e., epidermis. 



