In general the Arctopithecini and _Ateles_, _Mycetes_, _Lagothrix_ and _Pithecia_ among the Cebidae form a group containing a series of caecal transition types which lead up to the anthropomorphous type, illustrating the following conditions: (_a_) The inherent crescentic curve of the caecum, with the concavity directed toward the left, and carrying the apex of the pouch upward toward the lower border of the ileum and the ileo-colic junction. (_Hapalidae_, _Ateles_, _Lagothrix_.) (_b_) The reduction in caliber of the terminal part, foreshadowing by the pointed and narrow extremity of the pouch the appearance of the appendix in the anthropomorphous group. (_Hapalidae_, _Ateles_.) (_c_) The constriction at the level of the ileo-caecal junction, with the corresponding well-marked differentiation between caecum and colon in the interior. (_Ateles._) (_d_) The sharp bend in the pouch as it makes its turn upward and to the left, repeated in certain types of adult human caeca (cf. p. 247). (_Lagothrix._) (_e_) _Pithecia_ forms a transitive type between the blunt sacculated caeca of the Cynomorpha and the curved pointed pouches of the Cebidae, partaking of the characters of both. (_f_) The same character is seen in the caecum of _Mycetes fuscus_ the brown howler monkey (Figs. 449 and 450). =4. Cebinae.=--In the typical genus _Cebus_ the caecum is placed laterad to the small intestine which is in direct linear continuity with the colon. The pouch is slightly convoluted toward its termination, resembling in this respect and in its position relative to the lumen of the intestinal canal, the disposition of the parts in the cynoid carnivora. Figs. 453 and 454 show the structures in two typical species, _Cebus monachus_ and _C. leucophaeus_. [Illustration: FIG. 453.--_Cebus monachus_, capuchin monkey. Ileo-colic junction and caecum; dried preparation. (Columbia University Museum, No. 26.)] [Illustration: FIG. 454.--_Cebus leucophaeus_, capuchin monkey. Ileo-colic junction and caecum. (Columbia University Museum, No. 1467.)] D. ANTHROPOMORPHA. The caecum is large, sacculated, provided uniformly with a vermiform appendix. The pouch of the four anthropoid apes agrees in curve, direction, implantation of the appendix and the general arrangement of the vascular and peritoneal folds with the structure in the human subject. =1. Hylobates hoolock, Gibbon.=--Figs. 455 and 456 represent respectively the ileo-caecum of this animal in the ventral view, and from the left side with the ileum turned forward. The caecum is a globular rounded pouch of nearly uniform diameter, only slightly enlarged to the right of the root of the appendix which arises from its lowest part and is pendent. [Illustration: FIG. 455.--_Hylobates hoolock_, hoolock gibbon. Ileo-colic junction and caecum; ventral view. (Drawn from Columbia University Museum preparation No. 1530.)]