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of their being successfully treated. Besides, different parts have different functions, and the longer the tube—again within necessary limits—the greater scope is there for division of labour, and consequent economy. The comparative length of the alimentary canal is not the same in all animals by any means. Carnivorous animals, like the cat, whose

food is soft and easily digested, have a fairly short one. Herbivora, like the sheep, whose food is difficult to digest and mixed with much husk, which is wholly indigestible, have a comparatively very long one. Man, who is omnivorous, but eats less and more judiciously chosen food than either of the above classes, has one of medium length. But in all cases among the higher animals there is an attempt made to

obviate the necessity of increasing the length of the animal by coiling the tube within the body. The annexed diagram (7) illustrates this principle. It shows a schematic animal whose digestive canal is much longer than itself.

The digestive canal has, however, another function. The cells which compose it have not only to secrete