52 Chapter III 



protoplasm of the intestinal cells becomes filled with round vacuoles, 

 containing brown irregular concretions excreta which are expelled 

 into the intestinal cavity. 



This slow digestion of a substance usually so easily assimilable as 

 blood takes place entirely within the epithelial cells of the intestine. 

 Continuous microscopical observation demonstrates most clearly the 

 complete absence of any extracellular digestion of the blood corpuscles 

 in the intestinal content. 



[56] When goose's blood mixed with blue litmus powder is given to 

 Planarians, the coloured grains may be 

 found some hours afterwards inside the 

 epithelial cells of the intestine, but only 

 a few of the blue litmus granules change 

 colour, taking on a light violet tinge ; the 

 great majority retain their blue colora- 

 tion. It might be concluded from this 

 that in Planarians intracellular digestion 

 is effected in a neutral or nearly neutral 

 medium. If, however, the preparations 

 of intestinal cells gorged with goose's ,- 

 blood are treated with a 1% solution 

 of neutral red, we at once notice that 

 the red corpuscles and the vacuoles 

 which contain them are stained bright 



red, assuming a tint similar to that given FIG. 9. Portion of an intestinal 

 with picrocarmine staining (fig. 9). This ^J^' *** 

 colour reaction indicates, according to 



our researches on neutral red, an acid reaction, more feeble, however, 

 than that met with in Paramaecium and many other Protozoa. 



Macerations of Planarians in normal saline solution to which has 

 been added a small quantity of the red corpuscles of the goose's 

 blood exhibit in vitro a very distinct solvent action on these cor- 

 puscles, which become rounded and lose their haemoglobin, this latter 

 diffusing into the surrounding fluid, and at the close of the experiment 

 there remain simply the membranes and the nuclei of the corpuscles. 



The study of these Planarians shows us, then, that the food of these 

 animals undergoes exclusively intracellular digestion in a feebly acid 

 medium and by means of a soluble ferment, and it furnishes us with 

 proof that typical intracellular digestion is essentially a chemical process 

 due to the intervention of enzymes. Now there can be no question, here, 



