THE METABOLIC MACHINERY OF AMMALS 281 



Digestion in Lower Animals 



Digestion within the animal kingdom is primarily of two sorts, 

 intracellular taking place within the cell and extracellular which is 

 carried on outside the boundaries of the cell. Sometimes both types 

 of digestion occur in the same organism. The complexity of the 

 picture among one-celled forms may be appreciated when it is real- 

 ized that within the confines of a single cell are carried on all the 

 essential processes characteristic of a many-celled organism. 



Euglcna, for example, shows evidence of being a rather generalized 

 physiological type (see page 157). Within the group to which it 

 belongs three types of nutrition are found: (1) holophijtic nutrition 

 carried on by the aid of chlorophyll ; (2) saprophytic nutrition cor- 

 responding to that carried on by the chlorophyll-less molds and 

 fungi ; and (3) holozoic nutrition, involving the ingestion of solid 

 food particles, a type characteristic of animals. Both Ameba and 

 Paramecium are characterized by relatively simple intracellular 

 digestion, the potential food reaching the interior of the cell by 

 means of a food vacuole, the indigestible particles being egested 

 from the cell later. 



In sponges ingestion and digestion principally occur in the collared, 

 or choanoflagellate, cells where food vacuoles are formed and wastes 

 egested. The nutritive material is then passed from one cell to the 

 other and, according to Hegner, may be circulated to a certain extent 

 by wandering ameboid cells found in the middle region by a similar 

 intracellular action. 



In the coelenterates one first finds evidence of extracellular digestion. 

 Here a special layer of cells called the endoderm, which lines the prim- 

 itive gastrovascular cavity, is set aside. This cavity appears in Hydra 

 as a simple sac lined by cells possessing the ability to send out either 

 flagella or pseudopodia. Some of these cells are glandular and 

 secrete digestive enzymes which are passed into the gastrovascular 

 cavity, making digestion an extracellular process. A certain amount 

 of intracellular digestion does take place, however, since some of the 

 food particles are surrounded by pseudopodia and so brought within 

 the walls of the endodermal cells. 



Most of the parts of the digestive system found in v(^rtebrates are 

 represented in the earthworm (see page 189). The digestive system 

 of a crayfish will be discussed here as representative of the Arthro- 

 poda. Its food consists of such organisms as frogs, tadpoles, small 



