OCR Text |
Show 1902.] ADAPTATIONS IN DIPROTODONT MARSUPIALS. 25 somewhat shorter in the young than in the adult. The difference is, however, not so great as in the case of a marsupial foetus of the Koala already mentioned. Although the latter represented a younger stage *, the conclusion can thus be drawn that the milk diet of the young Petaurus differs, with regard to its composition and therefrom resulting influence on the intestine, less than it does in the case of the young Koala. It is also of interest to note that in the young Petaurus the caecum stands in quite open communication with the colon. That is because the sphincter is not needed yet for the purpose of prohibiting any indigestible remains from entering the caecum as in the adult. In Acrobates the arrangement of the connection between the small and the large intestine is the same as in the adult Petaurus. The ileum opens with its valve protruding like a mouthpiece into the colon ; and there is a very strong constriction between the latter and the caecum. Although the stomach in m y specimen of Acrobates was empty, I think it may be assumed that it lives on a similar diet to Petaurus; and at any rate the function of the caecum seems to be perfectly alike in both animals. The small intestine of Acrobates is comparatively shorter even than that of Petatirus. In the latter and in Phalanger the length of the small intestine is comparatively not much different. This may be understood as meaning that that part of the food-material which ought to be digested in the small intestine of Phalanger is not difficult to digest. It may chiefly consist of the juice and other contents of the soft parenchymatous cells of the fruit-pulp. This matter is, of course, more easily accessible for the digestive organ than is the material contained in the better protected cells of the leaves etc., which form the food of the animals considered above, and the small intestine of which, therefore, has become lengthened. As the last stage in this series, showing a different development of the intestine and especially of the caecum in accordance with the different diet, Tarsipes may be mentioned; this animal has, as already remarked, entirely lost its caecum, because such an organ is superfluous for a honey-eater. The general arrangement and structure of the dentition of these animals indicate that also with regard to those parts adaptations for different purposes have taken place. The dentition of Phascolomys is evidently most specialized. Its incisors are more reduced in number than in the others, the canines are absent, and the molars have persistent pulps. The latter are also curved in such a way that the upper molars are laterally concave and the lower ones are laterally convex. This development and shape 1 The Koala foetus was still naked. The young Petaurus was beginning to become hairy, so that, for instance, the dark vertebral stripe was well conspicuous, but the hairs of the tail were not yet prolonged. It had certainly not yet partaken of any other food than milk, because the mandibular incisors, although protruding 3 m m . from the sockets, had not cut the gum. |