SCALES AND TEETH 



23 



The mode of origin of the lungs as an unpaired divertic- 

 ulum of the gullet is in every sense similar to that of the 

 air-bladder. 



2. THE DERMAL DEFENCES OF FISHES 



The dermal defences of fishes include scales, spines, fin 

 rays, armour plates, and teeth, presenting in all a wide 

 range of calcified structures. They have usually an outer, 

 or surface layer of hard enamel-like texture and an inner 

 substance heavy, stout, and bone-like. The former is de- 

 rived from the outer layer of the skin (epidermis), the 

 latter from the derma. The relation of these structural 

 parts may be well seen in a section of shark skin which 

 passes through one of its minute limy cusps, or dermal 

 denticles (Fig. 20). The outer skin layer, E', originally 

 covered the denticle, which grew outward, papilla-like, 

 beneath it ; its inner surface, in contact with the outgrow- 

 ing papilla, secreted the enamel, E, and is known as the 

 enamel organ, EO: at the cusp, however, the epidermis is 

 early worn away. The bone-like substance of the tooth is 

 clearly formed in the lower (dermal) layer of the skin, D' : 

 it is formed by the calcification of the outer layers of the 

 tip and base of the dermal papilla, leaving a vascular cavity, 

 PC, within. This limy substance, "dentine," D, presents 

 microscopically a columnar "cancellated" structure; in 

 this and in its lack of bone cells it differs structurally 

 from true (cartilage) bone. 



The dermal denticle of the shark is certainly the sim- 

 plest form of a calcified skin defence : it appears to repre- 

 sent the ancestral condition of the various scales, teeth, 

 or bone plates which have been evolved in the groups of 

 fishes. It is usually of minute size, and studs closely the 

 entire surface of the skin, forming shagreen. In many 



