ii8 PHYSIOLOGY OF THE INVERTEBRATA. 



readily than one of gum ; and the same liquid will be ab- 

 sorbed more readily by one membrane, and less so by others. 

 Thus every membrane has a special power of absorption for 

 certain fluids, which it will take up in greater or smaller 

 quantity, according to their nature and composition. In all 

 cases, however, there is a natural limit to this quantity, 

 beyond which absorption will not continue. 



In the higher animals there is absorption by the blood- 

 vessels and absorption by the lacteals ; but, as already stated, 

 there are no distinct vessels in the Invertebrata set apart for 

 the function of absorption. In the lower Mollusra, Echino- 

 deriiKita, &c., the digested food is absorbed by the walls of 

 the alimentary canal. In the higher MoUusc(r and Arthropoda, 

 the digested food or nutritive fluids are absorbed by the blood- 

 vessels in the walls of the alimentary canal. In both of the 

 above cases, the two functions of absorption and digestion are 

 not completely differentiated from each other. 



In the Inverteljrata the digested food is brought into con- 

 tact or close relationship with the various tissues in three 

 ways : 



(i) The food particles (as in Amceha), during the process 

 of digestion, are brought into contact with the tissues (using 

 the term in its widest sense), that are to be nourished or 

 renovated by them. In this case there is a fusion of the two 

 functions of absorption and digestion. The digested food 

 immediately becomes tissue. 



(2) The digested food or nutritive fluid transudes through 

 the walls of the alimentary canal into the somatic or Iwdy 

 cavity, and is consequently absorbed by the walls of, and the 

 organs suspended in, that cavity. In this case, the nutritive 

 fluid passes through a transitory condition, in such a state 

 being known as the " chylaqueous " fluid. The so-called 

 chylaqueous fluid is found in the body cavity, and is never 

 enclosed in any distinct vessels ; it undoubtedly represents 

 the blood of the higher animals. 



(3) ^rhe digested food contained in the alimentary canal is 



