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 diges- 

 tive fluids are called enzymes, and each kind of enzyme 

 acts on only one class of food material. An animal's prac- 

 tical 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 in- 

 sects 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 ma- 

 terials in the stomach that the animal needs for its suste- 

 nance 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 intes- 

 tine and can so enter the closed cavity of the body. The 

 next problem is that of distribution, for still the food ma- 

 terials 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 



[in] 



