but in none is it reduced to so great an extent as in the Cetacea, in
which it is limited to a few small bristles confined to the neighbourhood
of the lips and nostrils, and often present only in the young,
or even the foetal condition.
Some kinds of hairs, as those of the mane and tail of the horse, persist throughout life, but more generally, as in the case of the body-hair of the same animal, they are shed and renewed periodically, generally annually. Many mammals have a longer hairy coat in winter, which is shed as summer comes on; and some few, which inhabit countries covered in winter with snow, as the Arctic fox, variable hare and ermine, undergo a complete change of colour in the two seasons, being white in winter and grey or brown in summer. There has been much discussion as to whether this winter whitening is due to a change in the colour of the individual hairs or to a change of coat. It has, however, been demonstrated that the senile whitening of human hair is due to the presence of phagocytes, which devour the pigment-bodies; and from microscopic observations recently made by the French naturalist Dr E. Trouessart, it appears that much the same kind of action takes place in the hairs of mammals that turn white in winter. Cold, by some means or other, causes the pigment-bodies to shift from the normal positions, and to transfer themselves to other layers of the hair, where they are attacked and devoured by phagocytes. The winter whitening of mammals is, therefore, precisely similar to the senile bleaching of human hair, no shift of the coat taking place. Under the influence of exposure to intense cold a small mammal has been observed to turn white in a single night, just as the human hair has been known to blanch suddenly under the influence of intense emotion, and in both cases extreme activity of the phagocytes is apparently the inducing cause. The African golden-moles (Chrysochloris), the desmans or water-moles (Myogale), and the West African Potamogale velox, are remarkable as being the only mammals whose hair reflects those iridescent tints so common in the feathers of tropical birds.
The principal and most obvious purpose of the hairy covering is to protect the skin. Its function in the hairless Cetacea is discharged by the specially modified and thickened layer of fatty tissue beneath the skin known as “blubber.”
Scales, &c.—True scales, or flat imbricated plates of horny material, covering the greater part of the body, are found in one family only of mammals, the pangolins or Manidae; but these are also associated with hairs growing from the intervals between the scales or on the parts of the skin not covered by them. Similarly imbricated epidermic productions form the covering of the under-surface of the tail of the African flying rodents of the family Anomaluridae; and flat scutes, with the edges in apposition, and not overlaid, clothe both surfaces of the tail of the beaver, rats and certain other members of the rodent order, and also of some insectivora and marsupials. Armadillos alone possess an external bony skeleton, composed of plates of bony tissue, developed in the skin and covered with scutes of horny epidermis. Other epidermic appendages are the horns of ruminants and rhinoceroses—the former being elongated, tapering, hollow caps of hardened epidermis of fibrous structure, fitting on and growing from conical projections of the frontal bones and always arranged in pairs, while the latter are of similar structure, but without any internal bony support, and situated in the middle line. Callosities, or bare patches covered with hardened and thickened epidermis, are found on the buttocks of many apes, the breast of camels, the inner side of the limbs of Equidae, the grasping under-surface of the tail of prehensile-tailed monkeys, opossums, &c. The greater part of the skin of the one-horned Asiatic rhinoceros is immensely thickened and stiffened by an increase of the tissue of both the skin and epidermis, constituting the well-known jointed “armour-plated” hide of those animals.
Nails, Claws and Hoofs.—With few exceptions, the terminal extremities of the digits of both limbs of mammals are more or less protected or armed by epidermic plates or sheaths, constituting the various forms of nails, claws or hoofs. These are absent in the Cetacea alone. A perforated spur, with a special secreting gland in connexion with it, is found attached to each hind-leg of the males of the existing species of Monotremata.
Scent-glands, &c.—Besides the universally distributed sweat-glands connected with the hair-system, most mammals have special glands in modified portions of the skin, often involuted to form a shallow recess or a deep sac with a narrow opening, situated in various parts of the surface of the body, and secreting odorous substances, by the aid of which individuals recognize one another. These probably afford the principal means by which wild animals are able to become aware of the presence of other members of the species, even at great distances.
To this group of structures belong the suborbital face-gland, “larmier,” or “crumen,” of antelopes and deer, the frontal gland of the muntjak and of bats of the genus Phyllorhina, the chin-gland of the chevrotains and of Taphozous and certain other bats, the glandular patch behind the ear of the chamois and the reed-buck, the glands on the lower parts of the legs of most deer and a few antelopes (the position of which is indicated by tufts of long and often specially coloured hair), the interdigital foot-glands of goats, sheep, and many other ruminants, the temporal gland of elephants, the lateral glands of the musk-shrew, the gland on the back of the hyrax and the peccary (from the presence of which the latter animal takes the name Dicotyles), the gland on the tails of the members of the dog-tribe, the preputial glands of the musk-deer and beaver (both well known for the use made of their powerfully odorous secretion in perfumery), and also of the swine and hare, the anal glands of Carnivora, the perineal gland of the civet (also of commercial value), the caudal glands of the fox and goat, the gland on the wing-membrane of bats of the genus Saccopteryx, the post-digital gland of the rhinoceros, &c. Very generally these glands are common to both sexes, and it is in such cases that their function as a means of mutual recognition is most evident. It has been suggested that the above-mentioned callosities or “chestnuts” on the limbs of horses are vestigial scent-glands; and it is noteworthy that scrapings or shavings from their surface have a powerful attraction for other horses, and are also used by poachers and burglars to keep dogs silent. The position of such glands on the lower portions of the limbs is plainly favourable to a recognition-taint being left in the tracks of terrestrial animals; and antelopes have been observed deliberately to rub the secretion from their face-glands on tree-trunks. When glands are confined to the male, their function is no doubt sexual; the secretion forming part of the attraction, or stimulus, to the other sex.
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Fig. 1.—Upper and Lower Teeth of one side of the Mouth of a Dolphin (Lagenorhynchus), as an example of the homoeodont type of dentition. The bone covering the outer side of the roots of the teeth has been removed to show their simple character. |
Dentition.—In the great majority of mammals the teeth form a definite series, of which the hinder elements are of a more or less complex type, while those in front are simpler. With the exception of the marsupials, a set of deciduous, or milk, teeth is developed in most mammals with a complicated type of dentition; these milk-teeth being shed at a comparatively early period (occasionally even in utero), when they are succeeded by the larger permanent series, which is the only other ever developed. This double series of teeth thus forms a very characteristic feature of mammals generally. Both the milk and the permanent dentition display the aforesaid complexity of the hinder teeth as compared with those in front, and since the number of milk-teeth is always considerably less than that of the permanent set, it follows that the hinder milk-teeth are usually more complex than the teeth of which they are the predecessors in the permanent series, and represent functionally, not their immediate successors, but those more posterior permanent teeth which have no direct predecessors. This character is clearly seen in those animals in which the various members of the lateral or cheek series are well differentiated from each other in form, as the Carnivora, and also in man.
In mammals with two sets of teeth the number of those of the permanent series preceded by milk-teeth varies greatly, being sometimes, as in marsupials and some rodents, as few as one on each side of each jaw, and in other cases including the larger portion of the series. As a rule, the teeth of the two sides of the jaws are alike in number and character, except in cases of accidental or abnormal variation, and in the tusks of the narwhal, in which the left is of immense size, and the right rudimentary. In mammals, such as dolphins and some armadillos, which have a large series of similar teeth, not always constant in number in different individuals, there may indeed be differences in the two sides; but, apart from these in describing the dentition of any mammal, it is generally sufficient to give the number and characters of the teeth of one side only. As the teeth of the upper and the lower jaws work against each other in masticating, there is a general correspondence or harmony between them, the projections of one series, when the mouth is closed, fitting into corresponding depressions of the other. There is also a general resemblance in the number, characters and mode of succession of both series; so that, although individual teeth of the upper and lower jaws may not be in the strict sense of the term homologous parts, there is a great convenience in applying the same descriptive terms to the one which are used for the other.
The simplest dentition is that of many species of dolphin (fig. 1), in which the crowns are single-pointed, slightly curved cones, and the roots also single and tapering; so that all the teeth are alike in form from the anterior to the posterior end of the series, though it may be with some slight difference in size, those at the two extremities being rather smaller than the others. Such a dentition is called “homoeodont” (Gr. ὄμοιος, like, ὀδούς, tooth), and in the case cited, as the teeth are never changed, it is also monophyodont (Gr. μόνος, alone, single, φύειν, to generate, ὀδούς, tooth). Such teeth are adapted only for catching slippery living prey, like fish.
