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, £’', originally 
covered the denticle, which grew outward, papilla-like, 
beneath it ; its inner surface, in contact with the outgrow- 
ing papilla, secreted the enamel, Z, and is known as the 
enamel organ, ZO: at the cusp, however, the epidermis is 
early worn away. The bone-like substance ofthe 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 
x 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- 
_ Pplest 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 
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