I20 PHYSIOLOGY OF THE INVERTEBRATA. 



it is due to the excitability* or irritabilityf of the cell, caused 

 indirectly by the presence of food particles. 



Speaking of the Rkizoiwda^ M. Richet says that "irritability 

 is their life complete." The presence of food particles excites 

 digestion and absorption, but only the digested particles 

 are absorbed. This power of selection is possessed by the 

 protoplasm of the cell ; it is a physiological property of 

 that complex substance whose composition has already been 

 alluded to in the early part of the work. 



In the compound Bhizopoda, only certain regions of the 

 sarcode take in food particles. " The food so ingested passes 

 through more or less of a compound Rhizopoda in a similar 

 fashion to that met with in the simple forms." 



In the Infusoria the iood particles may possibly undergo 

 a preliminary digestion in the short oesophagus (j\g. Para- 

 mcecium). After this the food gives rise to food vacuoles in 

 the sarcode. These food vacuoles undergo a rotatory move- 

 ment round the cell, just below the cuticular layer. " Only the 

 sarcode immediately in contact with the food vacuoles, as they 

 pass round, can be regarded as truly absorptive. Here is, 

 then, the first marking off of a region (only a region) of the 

 sarcode, whose work is that of absorbing nutrient materials 

 from the food, and transferring them to other parts of the 

 sarcodic body." 



The Porifera. 



In the Porifera the food particles, along with water, enter 

 through the inhalent apertures, and pass into the gastro- 

 vascular cavity, which is lined with flagellate cells ; but the 

 functions of digestion and absorption in the Porifera do not 

 differ very much from those occurring in the Rlnzopoda. 



The Ccelenterata. 

 In the Hydrozoa the function of absorption is somewhat 



* See Dr. Romanes' Mental Evolution. 

 t Richet's Ksnai dc Fsychologie Ginirale. 



