The more rapid and complete the digestion is the greater will be the amount of digested nutritive material at any given time in the intestine, and the greater will be the development of the absorbing structures. Hence the villi of the small intestine are especially large and prominent in the carnivora, while they are small and insignificant in herbivora and omnivora. Intestinal villi are found in nearly all mammals and in many birds. Fig. 300 shows the villi of the intestinal mucous membrane in a carnivore mammal (_Ursus maritimus_, polar bear) and Fig. 301 the same structures in the cassowary (_Casuarius casuarius_) in which bird they are very well developed. The villi are not confined to the two highest vertebrate classes, but are encountered also in the mucous membrane of the midgut in certain reptiles, notably the ophidia. [Illustration: FIG. 299.--Schematic sagittal section of abdomen to illustrate the intestinal branches of the abdominal aorta. The gastric and hepatic arteries are shown for the sake of convenience as arising together from the coeliac axis (_B_), hence the left and right gastro-pancreatic folds carrying these vessels appear fused at their beginning, separating the hepatic recess of the lesser peritoneal sac (_A_) from the cavity of the omental bursa (_C_).] [Illustration: FIG. 300.--Small intestine, of polar bear, _Ursus maritimus_. Mucous surface. (Columbia University Museum, No. 782.)] [Illustration: FIG. 301.--Duodenum, with entrance of pancreatic and biliary ducts and well-developed diverticulum Vateri in the cassowary, _Casuarius casuarius_ (Columbia University Museum, No. 1821.)] [Illustration: FIG. 302.--Mucous membrane of midgut of _Boa constrictor_. (Columbia University Museum, No. 1837.)] Fig. 302 shows the intestine mucous membrane of the boa constrictor with well-developed and prominent villous projections. Some birds, such as the snipes, herons and crows, have in place of the intestinal villi projecting folds of the mucosa, often arranged in a reticular manner. This type is prevalent in amphibia and fish (Fig. 112, distal segment of midgut). Collections of lymphoid tissue in the mucous membrane of the small intestine, either aggregated to form Peyer's patches (Fig. 309) or as solitary follicles, are only found in the two highest vertebrate classes, birds and mammals. In the former they appear scattered over the surface of the mucous membrane, in the latter they may be arranged in aggregations or regular rows. They are not secreting structures, but their exact function in absorption is not known. This lymphoid or adenoid tissue in certain forms is especially well developed at the ileo-colic junction, forming the _lymphatic sac_ of some rodents, as lepus (cf. Fig. 386). It is not confined to the small intestine, but is found in the large intestine as well. At times it appears especially well developed in the terminal portion of the caecal pouch (appendix), as in _Lepus_ (Fig. 388).