3. The ileo-colic junction is provided with a well-developed sacculated cæcal pouch derived from the proximal segment of the colon and divided in the interior by folds into several secondary compartments.

This type is found in some of the phytophagous lizards, as Iguana tuberculata (Figs. 326 and 327). The small intestine of this animal is of considerable length and of uniform caliber from the pylorus to the ileo-colic junction. The cæcum is a large sacculated pouch developed chiefly along the convex border of the large intestine opposite to the mesenteric attachment.

Fig. 327.—Iguana tuberculata, iguana. Mid-gut, ileo-colic junction, cæcum, and end-gut; dried preparation. (Columbia University Museum, No. 178.) Fig. 329.—Drawing taken from same preparation (No. 1321) to elucidate more clearly internal structure of cæcal pouch.
Fig. 328.—Iguana tuberculata, iguana. Ileo-colic junction and cæcum in section. (Columbia University Museum, No. 1321.) Fig. 330.—Cyclura teres, smooth-backed cyclura. Ileo-colic junction and cæcum in section. (Columbia University Museum, No. 1523.)

The examination of the interior of this pouch reveals a complicated structure (Figs. 328 and 329). Fig. 330 shows the same structures in a closely allied form, Cyclura teres. The entrance of the small intestine is guarded by an annular sphincter valve, whose central circular opening leads into a proximal compartment of the cæcum. This compartment is in turn separated from the remainder of the cæcal pouch by a second circular valvular fold with central opening. Beyond this valve the interior of the pouch carries a number of crescentic mucous folds, corresponding to the external constrictions between the cæcal sacculations. The entire pouch gradually diminishes in caliber and finally passes with a sharp angular bend into the terminal portion of the endgut. At this point the lumen of the canal is slightly diminished by a sphincter-like thickening of the muscularis, producing an annular projection of the mucous membrane. The entire cæcal pouch appears as a specialized segment of the large intestine interposed between the termination of the midgut and the terminal portion of the endgut, which latter is characterized by uniform caliber and increased thickness of the muscular walls.

The highly developed and complicated structure of the cæcal apparatus in Iguana and allied forms exemplifies very clearly the influence of vegetable food on the development of this segment of the alimentary tract when compared with the simple type of ileo-colic transition found in carnivorous lizards, as Heloderma ([Fig. 322]). Iguana subsists on leaves, fruits and other vegetable matter and the cæcal pouch is invariably found filled with the firmer and less digestible portions of this food. These are undoubtedly retained in the pouch by the series of valves and folds until digestion and absorption of all available nutritive material forwarded from the small intestine is completed. On the other hand Heloderma lives almost entirely on bird eggs, a concentrated and easily digested food. Consequently the ileo-colic junction in this lizard is exceedingly simple and rudimentary, marked merely by a slight external constriction, with an annular valve in the interior, and an increase in the caliber of the short hindgut, resembling the form found in many teleost fishes.

4. Finally in some Ophidians a typical lateral cæcal pouch of considerable dimensions is found connected with the endgut immediately beyond the ileo-colic junction.

An example of this reptilian type, closely resembling the corresponding structure in many Mammalia, is presented by Eunectes marinus, the anaconda, shown in Figs. 331 and 332.

Fig. 331.—Eunectes marinus, anaconda. Mid- and end-gut, with ileo-colic junction and cæcum. (Columbia University Museum, No. 72/1535.)   Fig. 332.—Eunectes marinus, anaconda. Mid- and end-gut, with ileo-colic junction and cæcum laid open. (Columbia University Museum, No. 1709.)

IV. ILEO-COLIC JUNCTION IN BIRDS.