One of the most complex of these structures which has been examined microscopically exists in the Marsupial Myrmecobius.[[5]] On the skin of the anterior part of the chest, just in front of the
sternum, is a naked patch of skin which is seen to be perforated by numerous pores. Besides the ordinary sebaceous and sweat glands there are a series of masses of glands, opening by larger orifices, which present the appearance of groups of sebaceous glands, and are of a racemose character; but the existence of muscular fibres in their coats seems to show that they should be referred rather to the sudoriparous series. Beneath the integument is a large compound tubular gland quite half an inch in diameter.
In Didelphys dimidiata there is a precisely similar glandular area and large underlying gland, the correspondence being remarkable in two Marsupials so distant in geographical position and affinities. Even among the Diprotodont genera there is something of the kind; for in Dorcopsis luctuosa and D. muelleri is a collection of four unusually large sebaceous follicles upon the throat, and in the Tree Kangaroo (Dendrolagus bennettii) there is the same collection of enlarged hair-follicles, though they are apparently somewhat reduced as compared with those of Dorcopsis. These are of course a few examples out of many.
It seems to be possible that the functions of these various glands is at least twofold. In the first place, they may serve, where predominant in one sex, to attract the sexes together. In the second place, the glands may be useful to enable a strayed animal of a gregarious species to regain the herd. It is perfectly conceivable too that in other cases the glands may be a protection, as they most undoubtedly are in the Skunk, from attacks. In connexion with the first, and more especially the second, of the possible uses of these glands, it is interesting to note that in purely terrestrial creatures, such as the Rhinoceros, the glands are situated on the feet, and would therefore taint the grass and herbage as the animal passed, and thus leave a track for the benefit of its mate. The same may be said of the rudimentary glands of Horses if they are really glands. The secretion of the "crumen" of Antelopes is sometimes deposited deliberately by Oreotragus upon surrounding objects, a proceeding which would attain the same end. One may even perhaps detect "mimicry" in the similar odours of certain animals. Prey may be lured to their destruction, or enemies frightened away. The defenceless Musk-deer may escape its foes by the suggestion of the musky odour of a crocodile. It is at any rate perfectly conceivable that the variety of odours among mammals may play a very
important part in their life, and it is perhaps worthy of note that birds with highly-variegated plumage are provided only with the uropygial gland, while mammals with usually dull and similar coloration have a great variety of skin glands. Scent is no doubt a sense of higher importance in mammals than in birds. The subject is one which will bear further study.
Nails and Claws.—Except for the Cetacea (where rudiments have been found in the foetus), the extremities of the fingers and of the toes of mammals are covered by, or encased in, horny epidermic plates, known as nails, claws, and hoofs.
The variety in the shape and development of these corneous sheaths to the digits is highly characteristic of mammals as opposed to lower Vertebrates. If we take extreme cases, such as the nail of the thumb in Man, the hoof of a Horse, and the claw of a Cat, it is easy to distinguish the three kinds of phalangeal horny coverings. But the differences become extinguished as we pass from these to related types. The nail of the little finger in Man approaches the claw-like form; and the hoofs of the Lama are almost claws in the sharpness of their extremities. On the whole it may be said that claws and hoofs embrace the bone which they cover, while nails lie only upon its dorsal surface. The form of the distal phalanx which bears the nail shows, however, two kinds of modification which do not support such a classification. When those phalanges are clad with hoofs or covered by a nail they end in a rounded and flattened termination. On the other hand, when they bear a claw they are themselves sharpened at the extremity and often grooved above.
The Marsupium.—It may appear to be unnecessary at this juncture to speak of the marsupial pouch, which is so usually believed to be a characteristic of the group Marsupialia. Rudiments of this structure have, however, been recently discovered in the higher mammals, and, as Dr. Klaatsch[[6]] has remarked, all researches into the "history of the mammals culminate in the question whether the placental mammals pass through a marsupial stage or not." We cannot, therefore, look upon the marsupial pouch as a matter affecting only the Marsupials, though it is true that this organ is at present functional only in them and in the Monotremata.
