396 GENERAL ZOOLOGY. 



from the rapid combustion of coal. In the case of a steam- 

 engine an engineer supplies the fuel, regulates the action 

 of the parts, and disposes of the waste. The animal must 

 be its own engineer. It must have the means of obtaining 

 fuel (food), of putting it in such position that the energy 

 produced by its oxidation can be utilized to its fullest 

 extent, and all waste can be properly disposed of. This 

 has led, in the first place, to the formation of a digestive 

 tract, in which the food is put in such shape as to be most 

 advantageously used by the organism. 



In the lowest animals (lowest Protozoa) we find that 

 the whole body (cell) serves as a digestive tract, and that 

 food can be taken in at any point on the surface. A 

 little higher (p. 162) an organ which we must call a mouth 

 is formed in the body, and this opening for the taking in 

 (ingestion) of food is found in all higher animals, except 

 a few parasites which, living on liquid food, need no such 

 opening. With larger animals a definite digestive cavity 

 or canal is formed, the lining of which has certain definite 

 work to perform. Most articles of food are insoluble as 

 taken into the body ; a bit of meat or starch can be soaked 

 indefinitely in pure water or may even be boiled for days 

 without passing into solution. In the digestive tract 

 juices are produced which alter these substances so that 

 they can be dissolved; and it is only when they are in 

 solution that they can pass through the walls of the 

 alimentary canal to those parts where they are to be 

 utilized. 



In the lower animals all parts of the digestive tract seem 

 able to act at once as formers of digestive fluids, and in 

 taking up of the dissolved food, but as we pass higher in 

 the scale complications of various kinds are introduced. 

 In the first place, we find certain organs, like the salivary 



