102 GENERAL PRINCIPLES OF ZOOLOGY. 



Vegetative Organs. 

 A. Organs of Assimilation. 



Assimilation Defined. If the term assimilation be used in its 

 widest sense, one must speak in this connection of all the con- 

 trivances in the animal body which render growth possible during 

 the period of progressive development, and, during mature life, 

 compensate for the loss of energy connected with each period of 

 functional activity, in order to preserve to the body its functional 

 powers. In each period of functional activity organic compounds 

 are oxidized. Compounds which are especially rich in carbon and 

 hydrogen (as well as some nitrogen and sulphur) and are poor in 

 oxygen are changed by oxidation into carbon dioxide, water, and 

 various nitrogenous products, like urea, uric acid, etc. A com- 

 pensation takes place, for not only is the useless substance 

 removed, but also compounds of oxygen and materials rich in 

 carbon are furnished to the tissues to replace the material oxidized. 



Assimilation in Animals. In lowly organized animals all the 

 processes connected with compensative assimilative changes take 

 place through the agency of one and the same organ, the digestive 

 tract; but in the higher animals, through specialization, normal 

 assimilation is a definite series of separate phenomena. Between 

 the lower and the higher animals there are evidently intermediate 

 conditions where specialization has halted at an earlier or a later 

 stage. 



Different Organs of Assimilation. Assimilation begins with 

 the presence of suitable food; the solid and liquid constituent 

 parts of the body must digest and incorporate this, i.e., it must 

 be altered so that it can be absorbed and distributed to the tissues. 

 All this takes place through the agency of the digestive tract, 

 which is provided with accessory organs, the digestive glands; the 

 digestive tract likewise removes all matter remaining undigested 

 (the faeces). The necessary oxygen, gaseous food, so to speak, is 

 usually taken, however, by particular parts of the body, the 

 respiratory organs, the gills or lungs. The oxygen and the 

 digested (consequently liquefied) organic and inorganic compounds 

 must further be distributed in the body to the organs and tissues 

 according to their needs. Therefore there are usually blood- 

 vessels or circulatory organs, which permeate the body in all 

 directions. But the tissues need not only a means of obtaining 

 but also of getting rid of certain compounds. The accumulation 



