Skin and Exoskeleton

Hairs.—Hairs are only found in the mammalian class, and are divided into the long tactile bristles or vibrissae and the smaller hairs which maintain the warmth of the body. In some animals the hair of the body is composed of long, stiff hairs, which are probably specialized for protective purposes, and short, soft hairs, which form the fur and keep in the warmth. Sometimes these long hairs are greatly enlarged and hardened to form pro tective spines as in the porcupine, hedgehog, spiny mouse and spiny ant-eater (Echidna).

Horns.—Horns are of three kinds: (I) antlers, (2) hollow horns and (3) hairy horns of the rhinoceros.

Antlers are growths of true bone and, except for their very vas cular covering of skin (velvet), are not exoskeletal structures.

They grow with great rapidity, and in the deer family are renewed each year. As soon as their growth is finished the skin covering dries up and strips off. The small horns of giraffes are also bony structures though permanent.

The hollow horns of the ruminants (Bovidae) are cases of har dened epidermis which fit over a bony core and are permanent.

They are found in both sexes, and in this differ from the antlers of the deer, which, except in the reindeer, are confined to the male. In the prongbuck (Antilocapra) the hollow horns are shed peri odically.

The hairy horns of the rhinoceros are a mass of hairs cemented together by cells. The hairs grow from dermal papillae, but differ from true hairs in not being sunk into hair follicles.

Claws and Hoofs.—These are modifications of nails, but whereas in nails and claws the structures are confined to the dor sal aspect of the digits, in hoofs they spread to the plantar surface as well. It has been shown in the embryological section of this article that the nail appears at the very tip of the digit, and in this position it remains in many amphibians, e.g. giant salamander, while in hoofed mammals it develops both ventrally and dorsally. In the Felidae the claws are retractile, but the real movement occurs between the middle and terminal phalanges of the digits.

Spurs.—Spurs are quite distinct from nails and claws; they are very common in birds as horny epidermal sheaths covering bony outgrowths of the radial side of the carpus, metacarpus or meta tarsus. The spur-winged goose has a carpal spur ; in the screamers (Palamedea and Chauna) the spur or spurs are metacarpal, while in many gallinaceous birds (e.g. common fowls and pheasants) metatarsal spurs are found. In the mammals the male monotremes (Echidna and Ornithorhynchus) have spurs attached to an extra ( ? sesamoid) bone in the hind leg, perforated for the duct of the already mentioned poison gland.

Beaks.—Certain fishes belonging to the family Mormyridae have a fleshy prolongation of the lower lip, and are hence termed beaked fishes. In the Amphibia Siren and the tadpoles of most Anura (frogs and toads) have small horny beaks. In the Reptilia horny beaks are found in the Chelonia while in birds beaks are constant and replace the teeth in modern species. In mammals a horny beak is found in Ornithorhynchus, though it coexists with true teeth in the young and with horny pads in adult specimens. In all these cases the beaks are formed from cornified epidermal scales.

Baleen.

The baleen which is found in the mouths of the Balaenidae or whalebone whales is a series of flattened triangular horny plates arranged on either side of the palate. The inner edges and apices of these are frayed out into long fibres which act as strainers. In Balaena mysticetus, the Greenland whale, there are nearly four hundred of these plates, the longest of which often exceed ten feet. In its development baleen resembles rhinoceros horn in that it consists of a number of epidermal hair-like fibres cemented together and growing from dermal papillae, though not from true hair follicles.

For further details and literature see R. Windersheim, Com parative Anatomy of Vertebrates, translated by W. N. Parker (London, 1907) ; S. H. Reynolds, The Vertebrate Skeleton (Cam bridge, 1897). (F. G. P.)