Chap. 1 1 FOODS AND NUTRITION 177 



devices, animals get their particular foods into their mouths, by pulling, push- 

 ing, cutting, and squeezing. Hunting and eating occupy most of the lifetime 

 of animals. Compare the winning and eating of food by all human beings. 



Essentials of Digestion 



Digestion is a series of physical and chemical changes by which food is 

 prepared for assimilation in protoplasm. Physically it is the breaking and mix- 

 ing of food; chemically it is the process of changing large organic molecules 

 into smaller ones through the action of hydrolyzing enzymes. Enzymes are not 

 only essential to digestion but to all other chemical activities of a living organ- 

 ism. In all multicellular animals and in many protozoans digestion occurs in a 

 cavity, a temporary one in the ameba, a sac in hydra, a tube in many inverte- 

 brates and in all vertebrates. Among the tools of digestion are beaks, teeth, 

 muscles, and secretions. 



Digestive Cavities and Tlieir Accomplishments 



Most of the multicellular animals contain relatively spacious digestive cavi- 

 ties (Fig. 11.6). In hydras, jellyfishes, corals, planarians, and others, it is a sac 

 with but one opening. In the great majority of animals it is a tube, the alimen- 

 tary canal, with extraordinary variations of structure and function. Some of 

 them are adapted to other uses besides those concerned with food, such as the 

 respiratory chamber in the intestine of the nymphs of dragonflies. 



Successful developments in the alimentary canals of various animals are: 

 holding capacity, means of movement and physical breakup of food, means of 

 chemical breakup, extensive cell surface for absorption of digested food, and 

 means of eliminating undigested waste. Animals have to take their food and 

 drink when and where they find it and a capacious stomach to carry away as 

 much as possible is useful. The stomach of a yellow perch may hold fishes of 

 the catch of yesterday, of the day before, and of the day before that, each lot 

 in a different stage of slow digestion. Cows graze steadily through the summer 

 forenoon, swallowing grass into their storage stomachs and chewing it over at 

 their leisure as they rest under the trees in the afternoon (Fig. 11.14). An 

 arrangement like this might be a happy one for commuters who must rush 

 through breakfast and catch the train. The holding capacity of stomachs is a 

 social asset to termites, honey ants, and several other animals. The social and 

 political prominence of many persons has been frequently due to the elastic 

 capacity of their stomachs, and just as frequently they have come to grief 

 because of it. Within colonies of certain species of honey ants, the repletes, 

 continually overfed with honeydew, are useful to the community as living 

 storage tanks of food and drink. From time to time a hungry worker taps the 

 head of a replete which promptly spits a drop of honeydew into the waiting 

 mouth of the worker. 



