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 caeca of these birds (cf. p. 204). The separation of the small and large intestine is marked externally by the _caecum_, 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. [Illustration: FIG. 310.--_Petromyzon marinus_, lamprey. Entire alimentary canal below pericardium. (Columbia University Museum, No. 1575.)] 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 caecum, 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 aconstriction, 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. [Illustration: FIG. 311.--_Echelus conger_, Conger eel. Alimentary canal, stomach, mid- and hindgut, liver, and spleen. (Columbia University Museum, No. 1430.)] [Illustration: FIG. 312.--_Echelus conger_, Conger eel. Ileo-colic junction, opened. (Columbia University Museum, No. 1434.)] [Illustration: FIG. 313.--_Echelus conger_, Conger eel. Section of mid- and end-gut, with ileo-colic junction, hardened. (Columbia University Museum, No. 1349.)] 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).