GENERAL ORGANOLOGY. 123 



tion 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 into a condition in 

 which 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 the mass 

 remaining undigested (the faeces). Oxygen, gaseous food, 

 so to speak, is taken up, however, by particular parts of 

 the body, the respiratory organs, gills or lungs. The oxy- 

 gen and the digested (consequently liquefied) organic and 

 inorganic compounds must now be further distributed in 

 the body and be apportioned to the functioning organs 

 and tissues according to their needs. Therefore there are, 

 in addition, 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 of the oxidation 

 products arising from functional activity is injurious, even 

 to some extent poisonous, 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 em- 

 braced under the name " excretory organs." Excreta are 

 to be carefully distinguished from fceccs ; excreta are sub- 

 stances 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 absolutely never belonged 

 to the body, but have remained separated from the tissues 

 by the boundary of the epithelium of the digestive tract. 

 The gaseous oxidation product of the animal body, car- 

 bonic acid, 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 



