GENERAL ORGANOLOGY. 103 



of the oxidation products arising from functional activity is 

 injurious, to some extent even poisonous, to the organism; conse- 

 quently 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 fceces; excreta are 

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

 and, through oxidation, have become useless; while those sub- 

 stances 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 boundary of 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 oxygen necessary to life, these organs have a double function, 

 being, at the same time, excretory organs and organs for taking 

 up food. 



After this general survey, we must enter somewhat mor& 

 minutely into a discussion of the various systems of organs. 



I. The Digestive Tract. 



Archenteron or Primitive Digestive Tract. Since the taking 

 in of food and its assimilation are functions most important for 

 the well-being of the animal, it is to be expected that of all the 

 organs in the animal series the digestive tract should be formed 

 first, and also in almost every case should be earliest established in 

 the embryo. The fact that many worms (cestodes) and Crustacea 

 (Rhizocephala) have no digestive tract does not alter this state- 

 ment; for it can be definitely affirmed that, in adaptation to 

 special conditions of life, particularly parasitism, the digestive 

 tract has degenerated. The simplest multicellular, free-living 

 animals are merely simple or branched digestive pouches which 

 have only a single opening, functioning both as mouth and anus 

 (fig. 57). Such an animal has necessarily two epithelial layers, 

 one of which lines the digestive tract, the other covers the surface 

 of the body. These two fundamental cell-layers are called ento- 

 derm and ectoderm. In many coelenterates they are the only 



