1. The development of the colic mucous membrane in the form of a projecting fold or valve usually surrounding the lumen spirally (Fig. 465, _IV_, 1). The significance and phylogeny of this spiral fold has been considered above (cf. p. 193). Functionally this reduplication must be regarded as in general equivalent to the caecal apparatus proper, in producing an increased surface for secretion and absorption and in retarding the movement of intestinal contents. The caecal pouch evidently acts as a reservoir in which partly digested substances, mixed with the secretions of the small intestine, are retained while the slow processes of digestion and absorption, already inaugurated in the antecedent segment of the canal, are completed. It is reasonable to suppose that the system of projecting mucous folds and reduplications encountered in the colon beyond the caecum have a similar physiological import. Moreover, in certain forms the caecum itself is provided with a similar spiral mucous fold, as in the instances already mentioned of _Lepus_ among mammalia (Fig. 381) and of the Ostrich among birds (Fig. 341). We have seen above (cf. p. 193) that the spiral intestinal valve is encountered very early in the vertebrate series, in forms in which the alimentary canal is but slightly, or not at all differentiated, short and straight in its course. In these forms the evident purpose of the spiral fold is to retard the movement of the intestinal contents and to increase the area of the secretory and absorbing surface. As a structural modification possessing this character we saw the fold in the Cyclostomata, Selachians and Dipnoeans (Figs. 310, 466, 467 and 468) and in certain Ophidians (_Python_ and _Anaconda_, Figs. 331 and 469). Among Mammals it is found in certain Rodentia in two forms: (_a_) In some of the Muridae, as _Arvicola_ (Fig. 394), the mucous membrane of the large globular caecal pouch is smooth, but the proximal segment of the colon, immediately beyond the ileo-colic junction, develops the spiral fold (Fig. 465, _IV_, 2).