5. _Caecal pouches of the birds_ (Fig. 465, _I_, 5).--A continuation of the backward extension of the bilateral colic pouches leads to the production of the typical double avian caeca in a greater or lesser degree of development. Frequently the caeca differentiate more completely from the colon, appearing as pouches of varying capacity joined to the large intestine by a narrower neck. Figs. 334-341 show the well-developed pouches as they appear in representative avian types, while Fig. 333 illustrates the reduction of the caecal apparatus encountered in many carnivorous birds. 6. Among mammalia _Cyclothurus didactylus_ (Fig. 360), the little ant-eater, furnishes an example of double symmetrical globular caeca, 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_ caecal pouches, resulting in central median implantation of the small intestine and its direct continuity with the colon. II. Asymmetrical Development of a Single Caecal 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 caecal 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 caecum found in all the Monotremes (Figs. 345-347, Fig. 465, _I_, 3). In its general appearance the caecum of these singular animals bears a close resemblance to the caecal pouches of many birds.