This unique arrangement, which is not found in any other known vertebrate, may possibly be led back to a type-form encountered in certain saurians (see [p. 234]).
4. Suborder: Proboscidea.
The cæcum of the elephant is a very large sacculated pouch with rounded termination, illustrated in Fig. 379, taken from the Asiatic elephant.
VII. Order: Rodentia.
With the exception of a single group, the dormice (Myoxus) (Fig. 380), the rodents possess a well-developed cæcal apparatus.
In some forms the terminal portion of the pouch is reduced so as to constitute an appendix. Many of these animals, in addition to the cæcum proper, have the proximal colon elongated and coiled in a spiral, and in some this part of the large intestine is provided in the interior with a spiral mucous fold. This latter structure functions again to increase the extent of the mucous absorbing surface and to prolong the retention of substances undergoing slow digestion and absorption.
Typical examples of the capacious sacculated rodent cæcum, with a terminal pointed reduced segment, are afforded by Castor fiber, the beaver (Figs. 381 and [382]) and by Erethizon dorsatus, the Canadian porcupine ([Figs. 383] and [384]). [Figs. 385] and [386] show the ileo-colic junction, cæcum and appendix in Lepus cuniculus, the rabbit. The interior of the cæcal pouch and of the proximal segment of the colon is provided with a complete spiral valve ([Fig. 387]), while the appendix is differentiated by the histological character of its mucous membrane which is studded with closely packed adenoid follicles ([Fig. 388]). A similar aggregation of lymphoid tissue is found in this animal at the ileo-colic junction forming the s. c. saccus lymphaticus ([Fig. 387]).
Fig. 381.—Castor fiber, beaver. Ileo-colic junction, cæcum, and proximal colon; ventral view. (Columbia University Museum, No. 1607.)