Fig. 348.—Didelphis virginiana, opossum. Ileo-colic junction and cæcum. (Columbia University Museum, No. 1533.)
The Didelphia are represented by numerous species, which are united by certain common anatomical characters of the reproductive organs and dentition to form the order of the Marsupialia. The individual species included within this order differ widely in abit, food, mode of locomotion, etc., and consequently exhibit great diversity in the structure of the skeletal and muscular systems and of the alimentary canal. With the exception of the Opossums inhabiting the new world, the families composing the order are confined to the Australian continent and the adjacent islands. In respect to the alimentary tract in general and the ileo-colic junction in particular, we are evidently dealing with a group of animals which, while they retain the common characters above indicated as uniting them in the marsupial order, yet have in the structure of their digestive canal adapted themselves to widely divergent conditions of food supply and environment. Consequently within the confines of this single and largely isolated order, we encounter nearly all the types of cæcum and ileo-colic junction which are found among the remaining mammalia. The group in its individual representatives has passed, so to speak, through the different stages of development and evolution which, on a very much larger scale, are exhibited by the remaining mammalian orders.
We can, independently of the systematic zoölogical classification, arrange the forms composing the order under the following types:
1. Forms with large well-developed simple cæca, of uniform caliber, with rounded globular termination.
This type is encountered among the herbivorous Marsupials, such as the opossums, kangaroos and wallabys. Fig. 348 shows the structures in Didelphis virginiana, the common opossum, Fig. 349 in a small species of opossum from Trinidad, and Fig. 350 the same parts in Halmaturus derbyanus, the rock wallaby.
2. Forms with enormously developed sacculated cæca, coiled spirally, with or without additional convolutions of the proximal colon; the terminal portion of the cæcal pouch diminishes in caliber to form a pointed appendage.
This type of cæcum characterizes the Phalangeridæ or Phalangers and the Phasolarctidæ.
Examples are shown in Figs. 351 and 352, representing the structures in Trichosurus vulpinus, the vulpine phalanger, and Phascolarctos cinereus, the koala.