GENERAL ORGANOLOGY 



93 



oxygen necessary to life, these organs have a double function, l)eing, at 

 the same time, excretory organs for taking up food. 



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 the digestive 

 tract should be formed first. The fact that many worms (cestodes) and 



Fig. 5S. Fig. 50. 



Fig. 58. — Longitudinal section through the nutritive polyp of a siphonophore 

 (after Haeckel). 0, mouth-opening; en, entoderm; ek, ectodern. 



Fig. 5g. — Stenostoma leiicops, in division, a, ectodermal fore-gut, at a' forming 

 anew for the hinder animal; m, the bhndly ending entoderinal mid-gut, c, ectodermal 

 ciUated epithelium; g, ganglion with ciliated pit; w, water-vascular canal; g', ganglion 

 of the hinder animal. 



Crustacea (Rhizocephala) have no digestive tract does not alter this 

 statement; for it can be definitely affirmed that, in adaptation to para- 

 sitism, the digestive tract has degenerated. The simplest multicellular, 

 free-living animals are merely simple or branched digestive pouches 



