Fig. 440 shows the structures in _Hapale jacchus_, one of the marmosets, and Fig. 443 illustrates the typical caecum of this form in _Ateles ater_, the black-handed spider monkey. 2. The caecum and appendix of man and of the anthropoid apes can be regarded as a reduction form of this type (Fig. 465, _III_, _A_, 2). Arrest of development of the terminal portion converts the distal segment of the caecal pouch into an appendix whose relation to the apex of the funnel-shaped proximal segment or caecum proper is seen in its pure form in the human embryo (Figs. 512 and 525). With the further development of the caecum the sharper demarcation between it and the appendix results (Figs. 517 and 518). The displacement of the root of the appendix cephalad and to the left, toward the lower margin of the ileo-colic junction, as it is usually seen in adults, is due to the relatively greater growth of the right terminal sacculation of the caecum compared with the left (cf. types of caeca, p. 248). Throughout these changes the initial crescentic curve of the caecum, turning its concavity upwards and to the left, can be recognized by tracing the course of the longitudinal colic muscular bands. The caeca and appendices of the anthropoid apes present the same characters. The structures in the orang, chimpanzee, gorilla and gibbon are shown in Figs. 455-464. B. The AEluroid and Arctoid groups of the Carnivora and the Pinnipedia constitute a very complete and instructive series illustrating the gradual reduction of the caecum from the capacious pouch of the primitive type and its final complete elimination from the organism (Fig. 465, _III_, _B_). In _Hyaena_ (Fig. 416), the large caecum with undiminished caliber of the terminal portion persists in its full development, as seen in the Marsupials furnishing the fundamental type (Fig. 465, _III_). The same type of caecum is found in the lion (Fig. 417), the only true cat in which the caecal apparatus has not undergone extensive reduction. Phylogenetically the presence of a capacious and uniform caecal pouch in these two animals is exceedingly important and indicates that this type of caecum represents the ancestral form common to the aeluroid carnivore group, which, in the remaining living representatives, has become reduced in response to the influence which the character of the food has on the structure of this portion of the intestinal canal. The two instances of persistence of the primal type are all the more important as exceptions to the rule which is otherwise universal throughout the group. 1. The first example of this reduction (Fig. 465, _III_, _B_, 1) is encountered in the Aard-Wolf, _Proteles lalandii_, a near relative of hyaena (Fig. 406). The caecum in this animal is considerably shortened, although still of fairly large and uniform caliber.