Fig. 354.—Phascolomys wombat, wombat. Ileo-cæcum and appendix. (Columbia University Museum, No. 1508.)
5. Forms with simple direct ileo-colic junction without cæcum.
In the purely carnivorous Marsupials, comprising the family of the Dasyuridæ, the reduction of the cæcal apparatus, foreshadowed by the appearance of the distal rudimentary segment as a vermiform appendix in the wombats, has been carried to the complete elimination of the pouch. The ileo-colic junction in these animals is simple, marked externally by a circular constriction and internally by an annular valve. The colon forms a very short terminal segment of the alimentary canal. The parts are shown in Fig. 355 in a typical representative of the family, Dasyurus viverinus, the Tasmanian devil.
Fig. 355.—Dasyurus viverinus, dasyurus, Tasmanian devil. Intestinal canal. Ileo-colic junction. (Columbia University Museum, No. 1463.)
The structural modifications encountered in the digestive tract of these carnivorous Marsupials can properly be regarded as the result of their habitual diet, and we will meet with analogous and identical examples of cæcal reduction in the typical Carnivores among the placental mammals (cf. [p. 212]).
III. Subclass: Monodelphia.
III. Order: Edentata.
In all probability the Sloths, Ant-eaters and Armadillos composing this order represent a highly specialized remnant of an ancient group now largely extinct. In respect to the ileo-colic junction the Edentates may be arranged in two groups which offer, within the limited number of existing species, a very complete transitional series.
I. Symmetrical Type of Ileo-colic Junction.