6. Among mammalia _Cyclothurus didactylus_ (Fig. 360), the little ant-eater, furnishes an example of double symmetrical globular cæca, connected with the colon by a narrow neck (Fig. 465, _I_, 6). Reference to the schema given in Fig. 465 will show that the types heretofore examined all have the following common character:
They appear derived from the primitive type by a differentiation in the caliber of the gut and by the gradual development of _symmetrical bilateral_ cæcal pouches, resulting in central median implantation of the small intestine and its direct continuity with the colon.
II. Asymmetrical Development of a Single Cæcal Pouch, Lateral to the Ileo-colic Junction, Mid- and Endgut Preserving Their Linear Continuity. (Fig. 465, II.)
In the second general group the symmetry of the ileo-colic junction is disturbed. The following types are encountered, forming a series of successive stages:
1. The increase in the caliber of the large intestine is chiefly marked along the border opposite to the mesenteric attachment, resulting in a greater degree of convexity in this part of the intestinal wall (Fig. 465, _II_, 1). Among Reptilia this condition is found in the ileo-colic junction of some of the pond-turtles, as _Pseudemys elegans_ (Fig. 323), while a mammalian example is furnished by the three-toed sloth, _Arctopithecus marmoratus_ (Fig. 363).
2. An increase of this lateral extension of the colon leads to the formation of a single lateral cæcal pouch (Fig. 465, _II_, 2) such as is seen in another Edentate, _Tamandua bivittata_ (Fig. 364), among Mammalia, and in certain Ophidians among Reptiles, as in the _Anaconda_ (Figs. 331 and 332).
3. Prolongation of the pouch and reduction in caliber lead to the formation of the slender lateral cæcum found in all the Monotremes (Figs. 345-347, Fig. 465, _I_, 3). In its general appearance the cæcum of these singular animals bears a close resemblance to the cæcal pouches of many birds.
4. Direct continuity of small and large intestine, with lateral colic cæcum, extending along the convex free border of the terminal ileum and slightly convoluted at the extremity (Fig. 465, _II_, 4), characterizes the entire group of the _Cebidæ_ among the new-world monkeys. The cæcum in these animals is a comparatively long pouch, nearly equalling in caliber the remainder of the intestine, occupying a distinctly _lateral_ position, with the terminal portion rounded and slightly recurved (Figs. 453 and 454).