INTEGUMENT. 27 
ing place at the surface of the stratum Malpighii. . In turtles and 
alligators there is a gradual wearing away of the surface. 
Closely allied to scales are claws, hoofs and nails (fig. 18). A 
claw may be regarded as a cap of the tip of a digit, formed by two scales 
one dorsal (unguis), the other ventral (subunguis). Of these the 
unguis is the more important. It grows continually from a root, and in 
mammals is forced forward over its bed. In the claw (B) the unguis 
is curved both transversely and longitudinally, the subunguis forming 
its lower surface. In the human nail (A) it is nearly flat in both direc- 
tions and the subunguis is reduced to a narrow plate just beneath the 
Fic. 18.—Diagrams of (A) nails, (B) claws, and (C) hoofs, based on Boas. ¢, unmodified 
epidermis; 7, unguis; s, subunguis. 
tip of the nail. In the hoof (C) the unguis is rolled around the tip of. the 
toe, while the subunguis forms the ‘sole’ inside it. The ‘frog’ is 
the reduced ball of the toe which projects into the hoof from behind. 
The integument presents many different conditions in the separate 
groups of vertebrates, and so details are best given under the special 
heads. 
FISHES.—The aquatic life renders the epidermis of fishes soft and 
cornifications of it are comparatively rare, among them the peculiar 
‘pearl organs’ which appear in the skin of some teleosts at the breed- 
ing season. Glands, on the other hand, are abundant. These are 
unicellular and multicellular mucus glands of different shapes in the 
epidermis, the secretion of which furnishes the slime on the surface. 
Some elasmobranchs and a number of teleosts have poison glands, 
usually in close relation to the spines of the fins. The elasmobranchs 
also have large glands in the ‘claspers’ of the males, but their purpose 
is not well understood. 
