. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. 266 Biology of the Vertebrates of the food waste is not even taken into the mouth. Instead the stomach is everted from the mouth in feeding and enwraps the food or prey so com- pletely that the indigestible parts are left behind when the stomach is withdrawn, leaving no residue to be passed out at the anus. Worms and caterpillars may well be described as perambulating diges- tive tubes, with the important mouth end pointed toward a food-contain- ing world.
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. 266 Biology of the Vertebrates of the food waste is not even taken into the mouth. Instead the stomach is everted from the mouth in feeding and enwraps the food or prey so com- pletely that the indigestible parts are left behind when the stomach is withdrawn, leaving no residue to be passed out at the anus. Worms and caterpillars may well be described as perambulating diges- tive tubes, with the important mouth end pointed toward a food-contain- ing world. Directive sense organs cluster around this exploratory end of the food tube, informing it where to go. A vertebrate in reality is a double tube. The outer tube is the protective body wall, and the inner tube, the digestive canal. Between the two tubes is the body cavity, which makes possible within a limited space the storage of a digestive canal much longer and more efficient than the exterior of the animal would lead one to suspect. Thus, the knapsack for carrying the ra- tions is bestowed within the body instead of being carried outside. 2. Increase in Digestive Surface So long as the bulk of an animal's body remains small a straight diges- tive tube has an adequate internal surface to meet all alimentary demands. It is mathematically demonstrable, however, that while the surfaces of two homologous solids are to each other as -the squares, the masses are to each other as the cubes of their homologous dimensions. This means that the bulk of a growing animal increases more rapidly than its surface, with the inevitable result that a straight unmodi- fied digestive tube becomes inadequate to take care of the accompanying mass. This is particularly true in the case of herbivores, whose food is less concentrated than that of carnivores, and who consequently need di- gestive machinery adequate for handling a larger quantity of food in a given time. There are four general ways in w
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