124 



GENERAL PRINCIPLES OF ZOOLOGY. 



useless carbonic acid 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 in of food and its assimilation are functions most 



FIG. 54. 



FIG. 55. 



FIG. 54. Longitudinal section through the nutritive polyp of a Siphonophore. (After 

 Haeckel.) O, mouth-opening j en, entoderm ; ek, ectoderm. 



FIG. 55. Stenostoma leucops, in division, a, ectodermal fore-gut, at a' forming anew for the 

 hinder animal ; >, the blindly ending entodermal mid-gut ; e, ectodermal ciliated epithe- 

 lium ; f, ganglion with ciliated pit ; iv, water-vascular canal ; g' ', ganglion of the hinder 

 animal. 



* As a substitute for the somewhat cumbrous terms "digestive 

 tract," "alimentary canal," etc., the term g ut is coming into use among 

 biologists to designate the canal as a whole. 



