4. Colic pouch prolonged back on each side of the ileo-colic junction, producing symmetrical colic cæca ([Fig. 465], I, 4).
A growth of the colic tube cephalad, on each side of the junction with the midgut, leads to the formation of this type, characterized by the presence of two symmetrical globular cæcal pouches. In its simplest form this condition is illustrated by the double colic cæca of another armadillo, Dasypus sexcinctus ([Fig. 359]).
The bifid cæcal apparatus of the American manatee ([Fig. 366]) belongs to the same group.
5. Cæcal 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 cæca in a greater or lesser degree of development. Frequently the cæca 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 cæcal apparatus encountered in many carnivorous birds.
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: