160 



KUTBITI03T. 



oreathe, by a process palled Respiration. The nutritive fluids 

 are conveyed to every part of the body by currents, usually 

 confined in vessels, and which, as they return, bring back the 

 particles which are to be either renovated or expelled. This 

 circuit is termed the Circulation. The function of Nutrition, 

 therefore, combines several distinct processes. 



SECTION I. 



OF DIGESTION. 



312. DIGESTION, or the process by which the nutritive 

 parts of food are elaborated and prepared to become blood, is 

 effected in certain cavities, the stomach and intestines, or ali- 

 mentary canal. This canal is more or less complicated in the 

 various classes of animals ; but there is no animal, however 

 low its organization, which is destitute of a digestive sac. 



[ 313. In the Hydraform POLYPIFERA, as in the common 

 fresh-water polype (Hydra viridis], the body consists of a diges- 

 tive sac, with a row of simple tentacula disposed around the 

 mouth, fig. 170. When the polype is watching for its prey 

 Fig. 170. it remains expanded, with its tentacula 



widely spread in all directions, to seize 

 a passing victim. No sooner does a 

 larve, or worm, or crustacean, impinge 

 upon one of these organs, than it is 

 arrested in its course as if by some ma- 

 gical influence : it appears fixed to the 

 almost invisible thread, and in spite of 

 its efforts, is unable to escape. The 

 prey, seized in this manner, and repre- 

 sented in fig. 1 70, is conveyed into the sto- 

 mach (), which has the appearance of a 

 delicate film, stretched over the contained 

 animal. If we watch attentively the pro- 

 cess of digestion, we observe the outline 

 of the included victim gradually becom- 

 ing more indistinct : soon are the soft 

 The Hydra viridis. parts dissolved, and reduced to a fluid 

 mass; and if any hard parts remain, as the shells of Cypris or 

 Daphnia, these are expelled through the oral aperture. It is 

 impossible to say by what process the nutritive product of 



