1. Large capacious smooth caecal pouch of uniform caliber (Fig. 465, _IV_, 2). This form is met with in the Muridae among Rodents and is illustrated in Fig. 393 showing the caecum of _Mus decumanus_, var. _albinus_, the white rat. Fig. 392 represents the entire alimentary canal of the meadow mouse, _Arvicola pennsylvanicus_, and indicates the proportion which the caecal apparatus bears to the remainder of the intestinal tract. The typical caecum of the Ungulates is shown in Fig. 371, taken from _Capra aegagrus_, the bezoar goat, and in Fig. 372, taken from a preparation of _Boselaphus tragocamelus_, the Nilghai. 2. The caecal pouch is large, markedly crescentic in shape, sacculated, or provided in the interior with a more or less complete spiral valve, and reduced in caliber in the terminal segment, forming at times a pointed appendix (Fig. 465, _IV_, 3). This form is encountered typically among certain Rodents, as in _Castor fiber_, the beaver (Figs. 381 and 382), and _Erethizon dorsatus_, the Canadian porcupine (Figs. 383 and 384), but is not confined to this order. Thus caeca of very similar structure are found among the Marsupials, as in _Phascolarctos_ and _Cuscus_ (Fig. 352). In some of these forms the terminal reduction of the caecum is very marked, resulting in a long narrow segment of the pouch tapering to a sharp point. It is significant to note in this connection that in one member of the marsupial order, the wombat (_Phascolomys_), this tendency to terminal reduction of the pouch has led to the development of a caecum and appendix identical in structure and arrangement with the corresponding parts of man and the anthropoid apes (Fig. 354). This is merely another illustration of the fact, evidenced throughout the entire vertebrate series, that a primal type-form of caecal apparatus, in responding to the conditions which influence the development of structural modifications, will produce identical specific types in animals otherwise widely separated in the zoological series. Thus again the form of caecum under discussion, found in many Rodents and certain Marsupials, is encountered in the only Insectivore possessing a caecum (_Galeopithecus_) (Fig. 419), and in several _Lemuroidea_ among Primates (Figs. 420-425). II. Structural Modifications of the Proximal Segment of the Colon Analogous in Their Functional Significance to the Caecal Apparatus. In these forms, in addition to the caecal apparatus proper, certain accessory structural modifications of the adjacent large intestine are developed which possess the physiological significance of the caecal apparatus in general, since they serve to increase the extent of the intestinal mucous surface and to prolong the period during which the contents of the canal are retained for elaboration and absorption. These modifications, which appear most fully developed in certain Rodents and Ungulates, are of two kinds.