PHAGOCYTOSIS 185 



gelatin and blood fibrin, less markedly, egg albumin. Its optimum 

 temperature is 25 C. (77 F.) or slightly above, but it acts slowly as 

 low as 8 C. At 60 C. it is destroyed. When solutions of this enzyme 

 are treated with bacteria killed with chloroform, the cloudy fluid soon 

 becomes perfectly clear. It must be noted that Mouton did not suc- 

 ceed in digesting living bacteria with the extracted enzyme, but they 

 are digested in living amebas. This suggests that the bacilli are killed 

 by one enzyme and digested by another. The killing enzyme may be 

 poured out into the surrounding fluid from the living cells and when 

 the amebas are killed its formation, quite naturally, stops. The forma- 

 tion of the digestive enzyme also stops when the amebas are killed 

 and the extract contains only the excess already formed. 



Most infusoria depend, in part at least, on intracellular digestion. 

 Most of them accomplish this by acid secretions, while some supply 

 feebly alkaline fluids. While protozoa depend largely on intracellular 

 digestion, this function is not wanting in many multicellular organ- 

 isms. In many invertebrates the digestive cells of the intestinal tract 

 consist wholly of sessile phagocytes. This is largely true of sponges 

 and coelenterates. This is beautifully illustrated in the siphon sponges 

 which are true beasts of prey. They swallow entire Crustacea which 

 are taken into the alimentary canal where they are surrounded by, 

 whole groups of entodermal phagocytes, fusing into great protoplasmic 

 masses, and digested. The actinea seize their victims with their ten- 

 tacles and digest them after absorbing them into their mesodermal 

 cells. Mesnil has succeeded in extracting from these cells an enzyme 

 which digests fibrin and coagulates albumin in feebly acid, neutral and 

 feebly alkaline solutions. The digestion proceeds at from 15 to 20 C., 

 the temperature at which actinea live, but in vitro, it is most effective 

 at from 35 to 45 C. Higher temperature weakens its activity which is 

 wholly arrested at from 55 to 60 C. The actino-enzyme produces pep- 

 tone and amino-acid. In these animals, digestion seems to be wholly 

 intracellular. 



While intracellular digestion is common in the intestinal canal of 

 the lower invertebrates, it is replaced by extracellular intestinal diges- 

 tion in the higher invertebrates and in the vertebrates. The intestinal 

 epithelium loses its ability to project pseudopodia, engulf food and 

 digest it. These cells become glands which pour out enzymes and 



