GENERAL OIIGANOLOGT. 103 



of tlio oxidation jiroducts arising from functional activity is 

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

 quently tliey must 1)0 removed, and in a dissolved state tliey are 

 taken up by the blood-vascuhir apparatus, aud 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 arc 

 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 fasces were useless from the beginnino-, 

 and have never belonged to the body, but have remained separated 

 from the tissues l)y 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 more 

 minutely into a discussion of the various systems of organs. 



I. The Digestive Tract. 



Archenteron or Primitive Digestive Tract. — Since the taking 

 ill 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 embrj'O. The fact that manj' worms (cestodes) and Crustacea 

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

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

 special conditions of life, particularly parasitism, the digestive 

 tract has degenerated. The simj)lest 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 e2>itlielial layers, 

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

 (.)f the body. These two fundamental cell-layers are called ento- 

 derm and ectoderm. In many cu;lenterates they are the only 



