154 



ELEMENTS OF BIOLOGY 



some, especially those which feed on plants, require only compara- 

 tively simple apparatus for dislodging and breaking off their food 

 substances; for example, cattle. In general, animal foods are in 

 solid form, or at least in chemical forms that require chemical 

 change before the materials can be utilized by protoplasm or the 

 energy in the food released. The conversion of this r^LW material 

 into chemical and physical conditions that make it available to the 

 cells of the animal body comprises the whole process of digestion. 

 The details of the process difiFer in different forms; however, all 

 animal digestive systems are designed to accomplish this common 



COOP 



GIZZARD TYPHLOSOLE 



PHAPYNX WAU 



MOUTH PHAPYNX OESOPHAGUS INTESTINE ANUS 



Fig. 10 1. — Digestive system of the earthworm. 



result. They involve (i) machinery for obtaining and mechanically 

 breaking up the food; (2) enzymes and other chemical agents for 

 dissolving and altering the food chemically; (3) areas for the ab- 

 sorption of the finished products of digestion; (4) a mechanism for 

 conveying these products from the site of digestion to all cells of 

 the body. This fourth principle links the digestive system with the 

 circulatory system and will be considered farther on. 



Abrading Mechanisms. The first of these, mechanically 

 breaking up the food, is accomplished by various devices usually, 

 but not invariably, associated with that part of the digestive system 

 which receives the food. In the diploblastic Porifera and Coelen- 

 terata and in Platyhelminthes, a considerable proportion of the 

 digestive process is accomplished within the endodermal cells, that 

 is, intracellular, and no mechanisms are provided for grinding or 



