In some of the birds of prey (eagle) the proportion falls as low as 1:68 or 70.

Exceptions to the general rule are furnished by some of the herbivorous Cetaceans and by the Dugong (Halicore) in whom the large intestine is twice as long as the small. Again in the Ostrich the large intestine in one example measured 40′, while the length of the small intestine was only 22′. This unusual development of the large intestine indicates the necessity of retaining the food, which is bulky and difficult of digestion, until the elaboration is completed. The same significance belongs to the enormously developed cæca of these birds (cf. [p. 204]).

The separation of the small and large intestine is marked externally by the cæcum, when present, and internally by the valve of the colon. The details of the vertebrate ileo-colic junction will be considered in the following pages.

II. SERIAL REVIEW OF THE ILEO-COLIC JUNCTION AND CONNECTED
STRUCTURES IN VERTEBRATES.

I. FISHES.

In the Cyclostomata there is no differentiation between the mid- and hindgut. [Fig. 310] shows the entire alimentary canal of Petromyzon marinus, the lamprey, caudad of the pericardium.

In some fishes the midgut is differentiated from the hindgut by an external circular constriction, corresponding to an annular projecting fold of the mucosa in the interior which resembles the pyloro-duodenal valve. There is no cæcum, and the short hindgut empties into the cephalic and ventral aspect of the cloaca. Fig. 311 shows the entire intestinal tract of a Teleost fish, Echelus conger, the conger eel. The midgut, provided at the beginning with a short globular pyloric appendix (cf. [p. 119]), constitutes the longest individual segment of the canal. The hindgut, separated from the preceding by a constriction, is very short and of large caliber. [Fig. 312] shows the broad annular valve with central circular opening which separates mid- and hindgut in the interior, and [Fig. 313] the ileo-colic junction in section in the same animal.

Fig. 310.—Petromyzon marinus, lam­prey.Entire alimentary canal belowperi­cardium. (Columbia University Museum,No. 1575.)

Fig. 311.—Echelus conger, Conger eel. Alimentary canal, stomach, mid- and hindgut,liver, and spleen. (Columbia University Museum, No. 1430.)

Fig. 312.—Echelus conger, Conger eel. Ileo-colic junction, opened.(Columbia University Museum, No. 1434.)

Fig. 313.—Echelus conger, Conger eel. Section of mid- and end-gut,with ileo-colic junction, hardened. (Columbia University Museum, No.1349.)

Fig. 316.—Galeus canis, dog-shark, ♂. Genito-urinary tract and cloaca in situ.(Columbia University Museum, No. 1694.)

Fig. 314.—Gadus callarias, cod-fish. Ileo-colic junction. Intestineon each side opened, with probe passed through constrictedopening of ileo-colic valve. (Columbia University Museum, No. 1260.)

Fig. 315.—Pleuronectes maculatus,flounder. Ileo-colon, opened to showileo-colic valve. (Columbia UniversityMuseum, No. 1493.)

Fig. 317.—Accipenser sturio, sturgeon. Alimentary canal. (Columbia UniversityMuseum, Nos. 1826, 1827, and 1828.)

A similar type of ileo-colic junction is seen in other Teleosts, as in Gadus callarias, the cod ([Fig. 314]), Pleuronectes maculatus, the flounder (Fig. 315), and in some Ganoids, as Accipenser sturio, the sturgeon ([Fig. 212]). In some Selachians an appendicular diverticulum, the so-called “rectal” or “digitiform gland,” is found connected with the terminal segment of the gut near the entrance of the same into the cloaca ([Fig. 316]).

II. AMPHIBIA.