The Vertebrate Skeleton

by Sidney H. Reynolds

Epidermal Exoskeleton.

Hair, which forms the characteristic Mammalian exoskeleton, varies much in different animals, and in different parts of the same animal. A large proportion of mammals have the surface fairly uniformly covered with hair of one kind only. In some forms however there are two kinds of hair, a longer and stiffer kind alone appearing on the surface, and a shorter and softer kind forming the under fur. In most mammals hairs of a special character occur in certain regions, such as above the eyes, on the margins of the eyelids, and on the lips and cheeks, here forming the vibrissae or whiskers.

Sometimes as in Hippopotamus, Orycteropus and the Sirenia, the hair, though scattered over the whole surface, is extremely scanty, while in the Cetacea it is limited to a few bristles in the neighbourhood of the mouth, or may even be absent altogether in the adult. In most mammals the hairs are shed and renewed at intervals, sometimes twice a year, before and after the winter. The vibrissae or large hairs which occur in many animals upon the upper lip, and the mane and tail of Equidae are probably persistent.

In the hedgehogs, porcupines and Echidna certain of the hairs are modified and greatly enlarged, forming stiff spines. Similar spines occur in the young of Centetes, and in Acanthomys among the Muridae.

Several other forms of epidermal exoskeleton are met with in mammals, including:—

(a) Scales. These overlie the bony scutes of armadillos and occur covering the tail in several groups of mammals, such as beavers and rats. In the Manidae the body is covered by flat scales which overlap.

(b) The horns of Bovine Ruminants. These, which must on no account be confused with antlers, are hollow cases of hardened epidermis fitting on to bony outgrowths of the frontals. In almost every case they are unbranched structures growing continuously throughout life, and are very rarely shed entire. In the Prongbuck Antilocapra however they are bifurcated and are periodically shed. Horns are nearly always limited to a single pair, but the four-horned antelope Tetraceros has two pairs, the anterior pair being the smaller.

(c) The horns of Rhinoceroses. These are conical structures composed of a solid mass of hardened epidermal cells growing from a cluster of long dermal papillae. From each papilla there grows a fibre which resembles a thick hair, and cementing the whole together are cells which grow from the interspaces between the papillae. These fibres differ from true hairs in not being developed in pits in the dermis. Rhinoceros horns may be either one or two in number, and are borne on the fronto-nasal region of the skull. They vary much in length, the longest recorded having the enormous length of fifty-seven inches.

(d) Nails, hoofs and claws. In almost all mammals except the Cetacea, these are found terminating the digits of both limbs. Nails are more or less flattened structures, claws are pointed and somewhat curved. In most mammals the nails tend to surround the ends of the digits much more than they do in man. Sometimes the nail of one digit differs from that of all the others; thus the second digit of the pes in the Hyracoidea and Lemuroidea is terminated by a long claw, the other digits having flat nails. In the Felidae the claws are retractile, the ungual phalanx with claw attached folding back when the animal is at rest into a sheath, above, or by the side of the middle phalanx. In the Sloths and Bats enormously developed claws occur, forming hooks by which the animals suspend themselves. In Notoryctes the third and fourth digits of the manus bear claws of great size; similar claws occur in Chrysochloris, being correlated in each case with fossorial habits. The nail at its maximum development entirely surrounds the terminal phalanx of the digit to which it is attached, and is then called a hoof. Hoofs are specially characteristic of the Ungulata.

(e) Spurs and beaks are structures which are hardly represented among mammals, while so characteristic of birds. They are however both found in the Monotremata. In both Echidna and Ornithorhynchus the male has a peculiar hollow horny spur borne on a sesamoid bone articulated to the tibia. The jaws in Ornithorhynchus are cased in horny beaks similar to those of birds, and are provided with horny pads which act as teeth.