The ileo-colic junction in all the existing forms is direct, without angular bend at the entrance of the small into the large intestine. The caecum is a long narrow pouch, slightly dilated at the extremity, derived from the beginning of the colon and extending backward along the free margin of the small intestine. The caecum resembles in its general shape and structure the pouches seen in many birds, except that it is unilateral, while the birds normally have two symmetrical caeca. The caecum of _Ornithorhynchus anatinus_, the platypus or duck bill, is shown in Figs. 345 and 346, and that of _Echidna hystrix_, the spiny ant-eater, in Fig. 347. These two animals represent the two genera into which the order is divided. [Illustration: FIG. 345.--_Ornithorhynchus anatinus_, duck mole. Ileo-colon and caecum. (Columbia University Museum, No. 1499.)] [Illustration: FIG. 346.--_Ornithorhynchus anatinus_, duck mole. Ileo-colon and caecum. (Columbia University Museum, No. 1500.)] [Illustration: FIG. 347.--_Echidna hystrix_, spiny ant-eater. Ileo-colon and caecum. (Columbia University Museum, No. 1501.)] II. Subclass: Didelphia. II. Order: Marsupialia. 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 caecum 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 zoological classification, arrange the forms composing the order under the following types: 1. _Forms with large well-developed simple caeca, of uniform caliber, with rounded globular termination._