WAYS AND MEANS OF LIVING
speed up these reactions, for otherwise the animal would starve on a full stomach by reason of the slowness of its gastric service. The quickening substances of the digestive fluids are called enzymes, and each kind of enzyme acts on only one class of food material. An animal's practical digestive powers, therefore, depend entirely upon the specific enzymes its digestive liquids contain. Lacking this or that enzyme it can not digest the things that depend upon it, and usually its instincts are correlated with its enzymes so that it does not fill its stomach with food it can not digest. A few analyses of the digestive liquids of insects have been made, enough to show that their digestive processes depend upon the presence of the same enzymes as those of other animals, including man.
The grosser digestive substances, in cooperation with the enzymes, soon change all the parts of the food materials in the stomach that the animal needs for its sustenance into soluble compounds which are dissolved in the liquid part of the digestive secretions. Thus is produced a rich, nutrient juice within the alimentary canal which can be absorbed through the walls of the stomach and intestine and can so enter the closed cavity of the body. The next problem is that of distribution, for still the food materials must reach the individual cells of the tissues that compose the animal.
The insect's way of feeding, of digesting its food, and of absorbing it is not essentially different from that of the higher animals, including ourselves, for alimentation is a very old and fundamental function of all animals. Its means of distributing the digested food within its body, however, is quite different from that of vertebrates. The absorbed pabulum, instead of being received into a set of lymphatic vessels and from these sent into blood-filled tubes to be pumped to all parts of the organism, goes directly from the alimentary walls into the general body cavity, which is filled with a liquid that bathes the inner surfaces of all the body tissues. This body liquid is called
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