Page:Popular Science Monthly Volume 17.djvu/622

604 While the whole digestive tract serves the purpose of a reservoir, the special reservoirs have indeed a digestive function, serving to delay the food, that it may be acted upon for a sufficient time by the chemical fluids. Thus the crop of a bird secretes a fluid which softens and prepares the hard grain for subsequent trituration and digestion.

—As the organ of digestion proper is the one most nearly universal, it consequently affords the finest example of specialization and development. From the improvised cavity of the amœba, there is a steady progress by minute steps to the complex apparatus of the mammals. Digestion is not more perfect, however, in the latter than in the former. The simple nutritive act of the amoeba is as perfect for itself as the differentiated process of the highest animals is for them. In the lowest animals, the function is single, and so simple that no special organ is necessary. As we rise in the animal scale, the function is divided into secondary functions, which require for their performance a corresponding number of special organs. Indeed, the complex functions of prehension, mastication, digestion, and circulation are only subdivisions of nutrition which begins in the lowest life as a single act. The present purpose, however, is not to trace the evolution of specialization of the digestive function further than to illustrate its general principles and methods, and present some of its peculiar and interesting features.

The tape-worm has no digestive organs whatever, having no use for them. A robber subsisting on the labors of its victim, it takes food in the same manner as a plant, by absorption from the outside. This is also the case with many lower protozoa.

The digestion of the amœba is only one remove higher than that of the tape-worm—with no permanent organs, but extemporizing a stomach from the skin as required. A step higher still we find the hydra, with a permanent body cavity serving the purpose of a stomach. But it is not distinctively a stomach, as it is the common organ