This spiral mucous fold is encountered in the straight intestinal canal of the Cyclostomata (Fig. 465, _IV_, 1, and Fig. 310), Selachians (Figs. 466 and 467) and Dipnoeans (Fig. 468). Phylogenetically it is a very old structure, for evidences of its existence are found in the fossil remains of some Elasmobranchs. In the Ostrich (Fig. 341) the enormously developed caeca possess the same spiral mucous fold in the interior. The direct combination of the caecum and spiral fold is again seen in certain mammalia, as in _Lepus_ (Fig. 387). In some Ophidians the same physiological purpose is served by the manner in which the convolutions of the long intestine are bound together by a subperitoneal arachnoid membrane. The lumen of the canal is thus made to assume a spiral course (Figs. 331 and 469). The mucous folds of the human intestine, both the valvulae conniventes and the crescentic folds of the large intestine, represent the same spiral valve, perhaps modified and influenced by the erect posture of man (Figs. 470-475). [Illustration: FIG. 470.--Human small intestine, opened to show valvulae conniventes. (Columbia University Museum, No. 1841.)] [Illustration: FIG. 471.--Human large intestine, showing colic taenia and plica. (Columbia University Museum, No. 1848.)] [Illustration: FIG. 472.--Human large intestine, opened and in section, showing colic plicae. (Columbia University Museum, No. 1847.)] [Illustration: FIG. 473.--_Cynocephalus anubis_, olive baboon. Large intestine, with cross-section showing colic taenia and plicae. (Columbia University Museum, No. 26/1168.)] [Illustration: FIG. 474.--Comparison of portion of human transverse colon with distal segment of rabbit's large intestine, showing same arrangement of longitudinal muscular bands (colic taenia) and colic sacculations. (Columbia University Museum, No. 1589.)] [Illustration: FIG. 475.--_Felis leo_, lion. Large intestine, with transverse section, showing smooth carnivore lumen, without sacculations or plicae. (Columbia University Museum, No. 1600.)]