92 GENERAL PRINCIPLES OF ZOOLOGY 



our scope, the entoparasites (cf. p. 157) take oxygen-containing com- 

 pounds from their hosts, form from them carbohydrates and reduce them 

 by a process analogous to fermentation into carbon dioxide and com- 

 pounds poor in oxygen. 



Assimilation in Animals. In lowly organized animals all the pro- 

 cesses connected with assimilative changes take place through the agency 

 of one and the same organ, the digestive tract; but in the higher animals r 

 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 provide! with accessory organs, 

 the digestive glands; the digestive tract likewise removes all matter remain- 

 ing undigested (the feces). 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-vascular or circulatory organs, 

 which permeate the body. But the tissues need not only a means of 

 obtaining new materials but also of getting rid of certain useless com- 

 pounds. The accumulation of the oxidation products arising from 

 functional activity is injurious to the organism; consequently they must 

 be removed, and in a dissolved state they are taken up by the blood- 

 vascular apparatus, and are brought to definite places for expulsion or 

 excretion. Fluid wastes are expelled by the kidneys of vertebrates, the 

 Malpighian vessels of insects, the water-vascular system of worms; these, 

 together with their accessory apparatus, are embraced under the name 

 excretory organs. Excreta are to be distinguished from J 'trees; excreta are 

 substances which have been a. part of the tissues of the body itself, and, 

 through oxidation, have become useless; while those substances which 

 constitute the faeces were useless from the beginning, and have never 

 belonged to the body, but have remained separated from the tissues by the 

 epithelium of the digestive tract. The gaseous oxidation product of the 

 animal body, carbon dioxide, is removed by the blood-vascular apparatus 

 through the agency of the respiratory organs. Since in the respiratory 

 organs there takes place an exchange of the useless carbon dioxide for the 



