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 en, entoderm ; ^.ectoderm. 



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

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

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

 animal. 



* As a substitute for the somewhat cumbrous terms "digestive 

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

 biologists to designate the canal as a whole. 



