INTEGUMENT 



31 



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 directions and the subunguis is reduced to a narrow plate just 

 beneath the 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. 



Strictly speaking the scales of fishes are tegumentary, but because 

 of their close relations to the skeleton they are best treated in that 

 connexion. 



Pig. 21. — Diagrams of (A) nails, (B) claws, and (C) hoofs, based on Boas, e, unmodi- 

 fied epidermis; u, unguis; s, subunguis. 



The integument presents many different conditions in the sepa- 

 rate 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 pecu- 

 liar 'pearl organs' which appear in the skin of some teleosts at the 

 breeding season. Glands, on the other hand, are abundant. There 

 are unicellular and multicellular mucus glands of different shapes in 

 the epidermis, the secretion of which furnishes the slime on the sur- 

 face (fig. 22). Some elasmobranchs and a number of teleosts have 

 poison glands, usually in close relation to the spines of the fins (fig. 

 23). The elasmobranchs also have large pterygopodial glands in the 

 'clampers' of the males, but their purpose is not well understood. 



Possibly the most striking of the epidermal organs are the lumi- 

 nous organs or photophores, which are most common in elasmobranchs 



