CHAPTER V. 



Absorption in the iNVERTEimATA. 



In Chapters III. and IV. the processes of digestion in the 

 Iiii'crtehrata were considered in detail. The dig-ested food 

 becomes tissue ; but before this is attained the said digested 

 food, which is still enclosed in the alimentary canal (if 

 present), must first pass through its walls and gain 

 entrance into the blood or tissues. This process is known 

 as xhsorptioii. 



The function of absorption in the Vcrtehrata is carried on 

 by a distinct set of vessels, but these are entirely wanting in 

 the Invertebrata. In the higher animals absorption takes 

 place partly in the stomach and partly in the intestine. 

 " The mucous membrane of the stomach and intestine con- 

 tains an abundant supply of capillaries ; the walls of these 

 vessels are only one cell thick ; consequently the soluble 

 peptones and sugar will diffuse readily into their interiors." 

 In the intestine the area of absorption is largely increased 

 by means of the villi in the Vcrtcbratu, and by means of the 

 typhlosole in those Invertebrates whose intestine is provided 

 with such an arrangement. 



There are no openings in the substance of the villi and 

 typhlosoles ; consequently the nutritive Huids pass directly 

 through their substance by a kind of transudation or imbibi- 

 tion (endosmosis). Every animal membrane will absorb 

 certain fluids with greater or less facility. Thus most of 

 them will absorb pure water more abundantly than a solution 

 of sodium or potassium chloride ; or a solution of sugar more 



