The order Marsupialia constitutes by itself the sub-class Didelphia, and forms with the Monotremata the division of the Non-placental Mammals. With the single exception of the genus Didelphys, which is American, all the Marsupialia belong to the Melanesian province; that is to say, they all belong to Australia, Van Die-men's Land, New Guinea, and some of the neighbouring islands.*
The following are the characters which distinguish the order:
The skull is composed of distinct cranial bones united by sutures, and they all possess true teeth; whilst the angle of the lower jaw is almost always inflected. The pectoral arch has the same form as in the higher Mammals, and the coracoid no longer reaches the anterior end of the sternum. All possess the so-called "marsupial bones," or cartilages, attached to the brim of the pelvis. The corpus callosum is very small, and has been asserted to be absent. The young Marsupials are born in a very imperfect condition, of very small size, and at a stage when their development has proceeded to a very limited degree only. (In the Kangaroo the period of gestation is only thirty-nine days, and in the Didelphidae it is Only fifteen or seventeen days.) There is no placenta or vascular communication between the mother and foetus, parturition taking place before any necessity arises for such an arrangement. As the young are born in such an imperfect state of development, special arrangements are required to secure their existence. When born, they are therefore, in the great majority of cases, transferred by the mother to a peculiar pouch formed by a folding of the integument of the abdomen. This pouch is known as the "marsupium" and gives the name to the order. Within the marsupium are contained the nipples, which are of great length. Being for some time after their birth extremely feeble, and unable to perform the act of suction, the young within the pouch are nourished involuntarily, the mammary glands being provided with special muscles which force the milk into the mouths of the young; while the windpipe is prolonged up to the posterior nares, where it is embraced by the soft palate, so that the food passes on each side of the trachea, and there is no risk of suffocation. At a later stage the young can suckle by their own exertions, and they leave the pouch and return to it at will. In a few forms there is no complete marsupium as above described; but the structure of the nipples is the same, and the young are carried about by the mother, adhering to the lengthy teats.
Fig. 359. - Lower jaw of the Wombat, viewed from behind, showing the strongly inflected angles of the jaw.
* One Kangaroo (Macropus Bruijnii) is found in the Indian Archipelago, along with five Phalangers, which differ from the Australian forms in having the tail partially or entirely naked or scaly. There are also Tree-kangaroos, and the curious Cuscus, distinguished by a prehensile tail, large eyes, and slow progression.
The so-called "marsupial bones" (fig. 360) doubtless serve to support the marsupial pouch and its contained young, but this cannot be their sole function, since they occur in the male Marsupials and also in the Monotremes, in which there is no pouch. It is believed by Owen that the function of the marsupial bones is to assist in the action of the mammae and testes, serving respectively as a fulcrum for the muscle spread over the mammary gland and for the cremaster.
The oviducts open into vaginal tubes which open into a urogenital canal; but this does not open into a "cloaca," though embraced by a sphincter muscle common to it and to the rectum. In other words, the vagina is separated wholly or in great part into two distinct tubes, but these open apart from the intestine. The testes are not abdominal throughout life as in the Monotremes, but are lodged in a scrotum. This, however, is placed in front of the penis, and not beneath the pubic arch as in most Mammals. From this unusual position of the scrotum, it is regarded by Owen as being the same structure as the marsupial pouch of the female, turned inside out.
Fig. 360. - One side of the pelvis of a Kangaroo, showing the " marsupial bones " (m). (After Owen.)
Though they form an extremely natural order, sharply separated from all the rest of the Mammals, the Marsupials form a large and varied group. In fact, this order, from being the almost exclusive possessor of a continent as large as Australia, has to discharge in the economy of nature functions which are elsewhere discharged by several orders.
The Marsupialia are divided by Owen into the two primary sections of the Diprotodontia and Polyprotodontia, comprising the following subordinate divisions: