In some of the Arctoidea, as Procyon and Nasua, the beginning of the colon just beyond the ileo-colic valve is bowed out opposite the mesenteric border indicating the original site of the eliminated cæcum, and recalling the arrangement of the intestine encountered above in Arctopithecus among the Edentates ([Figs. 363], [407], [412], and [465], III, B, 3). Moreover, in the same forms rudimentary vascular and serous folds around the ileo-colic junction, corresponding to similar structures found in connection with a well-developed cæcal apparatus in other mammalia, point to the former existence of a cæcum.
4. In the typical Ursidæ even these remnants and traces of a cæcal pouch have disappeared and the intestinal canal preserves a uniform caliber, without any differentiation of large and small intestine ([Figs. 414] and [415], [Fig. 465], III, B, 4).
C. The last subdivision of the third main group contains forms in which the large uniform pouch of the primal type appears moderately reduced in length and sacculated, terminating either in a blunt extremity or carrying a distal constricted and rudimentary segment as an appendage.
1. The first of these types is encountered in the Old World cynomorphous monkeys. In all of these animals the cæcal pouch is wide but comparatively short, of nearly uniform caliber and sacculated like the rest of the colon, of which it forms the direct caudal continuation ([Fig. 465], III, C, 1). The terminal portion of the pouch is usually blunt, globular and rounded ([Figs. 428], [430] and [431]), in a comparatively small number of forms slightly pointed ([Figs. 427] and [437]).
2. In the second group the terminal reduced portion persists either as a fairly distinct appendage, or in the form of a tapering pointed extremity into which the cæcal pouch proper is continued ([Fig. 465], III, C, 2). This type is encountered in certain non-ruminant Ungulates. An example of the first condition is furnished by the cæcal apparatus of the peccary (Dicotyles torquatus) ([Fig. 370]), while the structures in Tapirus americanus ([Fig. 377]) illustrate the second form.
IV. Cæcal Apparatus Combined with Structural Modifications of the Proximal Colon of Similar Physiological Significance. ([Fig. 465], IV.)
The fourth general mammalian group comprises forms in which the cæcal pouch is large, with or without terminal appendage, while in addition the large intestine develops structural modifications which possess the general functional significance of the cæcal apparatus. This highly developed and complicated structure of the alimentary canal indicates that the habitual food of these animals is bulky and difficult of digestion. Accordingly we find the group composed in main of the majority of the Ungulates and Rodents (with the exception of Myoxus), forms in which the diet under natural conditions is purely herbivorous. Other mammalian orders, however, also furnish representatives of this type of cæcal apparatus, the conditions as regards character and quantity of food habitually taken corresponding to those encountered among the Ungulates and Rodents. Thus the Phalangers among Marsupials ([Fig. 352]), Galeopithecus ([Fig. 419]) as an exceptional form among the Insectivora, and certain lemurs among Primates ([Figs. 420]-[425]) present examples of a highly developed and specialized type of cæcal apparatus.
The intestinal tract of these forms must therefore be considered from two points of view:
I. The cæcum proper.