INTEGUMENT. 



2 7 



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 



FIG. 18. Diagrams of (.4) nails, (5) ckws, and (C) hoofs, based on Boas, e, unmodified 

 epidermis; n, 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. 



