Specifically, the caecum appears as a pouch or diverticulum in which food-substances, already partially digested and mixed with the secretions of the small intestine, are retained until their elaboration is completed and the nutritive value of the food ingested is secured for the organism. Consequently the most complicated and highly developed caecal apparatus is found among mammalia in the Herbivora, such as the Ungulates and Rodents, whose food contains a comparatively small amount of nutriment in ratio to its bulk, and hence requires considerable elaboration before absorption. On the other hand the caecum appears as a reduced or even rudimentary organ, or defaults entirely, in Carnivora whose food is concentrated and easily assimilated, containing only a small amount of non-nutritive material. The function of the caecal apparatus may be defined as follows: 1. It provides space for the retention of partly digested substances, and of such as are difficult of digestion, mixed with the secretions of the preceding intestinal segment, until the digestive elaboration is completed. 2. It increases the intestinal mucous surface for absorption, and may develop, in certain cases, special localized areas of lymphoid tissue. These two functional characters may be shared by other segments of the intestinal tract, which undergo corresponding structural modifications. It is only necessary to refer in this connection to the extreme morphological variations encountered in the stomach. The intestinal canal proper, however, in many instances exhibits structural peculiarities which possess the functional significance of the caecal apparatus. Thus the projection into the lumen of the canal of a series of mucous folds, or the development of a continuous spiral mucous valve, evidently serves the double purpose of prolonging the period during which the intestinal contents are retained, and of increasing the intestinal mucous surface for absorption. [Illustration: FIG. 466.--_Squalus acanthius_, dog-fish. Alimentary tract, spleen, pancreas. (Drawn from Columbia University Museum preparation No. 1405.)] [Illustration: FIG. 467.--_Galeus canis_, dog-shark. Alimentary tract opened, showing spiral intestinal valve. (Drawn from Columbia University Museum preparation No. 1429.)] [Illustration: FIG. 468.--_Ceratodus forsteri_, Australian lung-fish. Intestinal canal with spiral valve. (Columbia University Museum, No. 1645.)] [Illustration: FIG. 469.--_Python molurus_, Indian python. Mid-gut, distended and fenestrated to show spiral course of lumen. (Columbia University Museum, No. 725.)]