3. This leads in the third stage, represented by _Tamandua bivittata_, the Tamandua ant-eater (Figs. 364 and 365), to the development of a distinct lateral caecal pouch. I have had no opportunity of examining the structures in _Orycteropus_, but from the published descriptions[8] the large caecum of this animal would form the final link in this series. [8] Flower and Lyddecker, "Mammals, Living and Extinct," p. 209. IV. Order: Sirenia. Of the two living representatives of this remarkable mammalian order the dugong (_Halicore_) is described as possessing a single caecum, while the caecal pouch of _Manatus americanus_, the manatee, is symmetrically bifid at the extremity (Fig. 366). [Illustration: FIG. 366.--_Manatus americanus_, American manatee. Ileo-colic junction and bifid caecum. (Columbia University Museum, No. 673.)] V. Order: Cetacea. In the majority of the whales the ileo-colic junction is simple without caecum, as in _Physeter_, _Delphinus_, _Monodon_ and _Phocaena_ (Fig. 367). [Illustration: FIG. 367.--_Phocaena communis_, porpoise. Ileo-colic junction. (Columbia University Museum, No. 1007.)] A few forms have a small caecal pouch. VI. Order: Ungulata. The intestinal canal, in conformity with the herbivorous habit of the group, is uniformly provided with a large caecum, and in many forms the proximal segment of the colon immediately beyond the ileo-colic junction is more or less extensively coiled in a spiral manner. This arrangement is, without doubt, to be regarded as being functionally accessory to the caecal apparatus, in the sense of increasing very much the area of the secreting and absorbing surface and of prolonging the period during which food-substances, which are slow and difficult of elaboration, are retained in this segment of the alimentary canal. 1. SUBORDER: ARTIODACTYLA. A. NON-RUMINANTIA.--In the _Suidae_ the caecum is large and the spiral colon well developed (Fig. 368). [Illustration: FIG. 368.--_Sus scrofa foet_, foetal pig. Ileo-caecum and spiral colon _in situ_. (Columbia University Museum, No. 1111.)] In the peccaries (_Dicotyles_) the terminal portion of the caecal pouch is reduced, constituting a centrally implanted appendage. [Illustration: FIG. 369.--_Dicotyles torquatus_, collared peccary. Ileo-colic junction, caecum, and spiral colon. (Columbia University Museum, No. 60/1462)] [Illustration: FIG. 370.--_Dicotyles torquatus_, collared peccary. Ileo-colic junction and caecum, isolated. (Columbia University Museum, No. 1464.)] Fig. 369 shows the ileo-colic junction and spiral colon in _Dicotyles torquatus_, the collared peccary, and Fig. 370 the caecum and appendix of the same animal detached from the spiral colon. In the hippopotamus, on the other hand, the caecum is said to be absent. If this is the case the animal forms an isolated exception among the Ungulates. B. RUMINANTIA.--The caecum is very large and the spiral coil of the colon extensive.