738 COMPARATIVE PHYSIOLOGY. 



inducing changes in substances brought into contact with it. 

 Krukenberg succeeded in extracting a peptic ferment from 

 the plasmodium of " Flowers of Tan," but did not believe 

 that it could have a digestive function on account of the 

 alkalinity of normal protoplasm. Metchnikoff, however, 

 has demonstrated in some cases that the fluid of " food 

 vacuoles " is acid, and seems to hold that all digestion is due 

 to ferment action. Miss Greenwood has also demonstrated 

 an acid in the vacuoles of several Protozoa, and described 

 the process of digestion. In any case we must note that the 

 formation of ferments appears to be a characteristic of pro- 

 toplasm ; but that as we ascend in the scale of being these 

 ferments are more and more utilised in the digestive pro- 

 cesses, and tend to be limited to the walls and outgrowths 

 of the alimentary canal. We may note here (as is more fully 

 explained in the section on Comparative Pathology) that in 

 most animals certain cells retain the primitive Protozoan 

 capacity for taking up and digesting solid particles, while the 

 general body cells have lost it. 



It is a fact of common observation that in parasites the 

 alimentary canal tends to be absent or degenerate ; nutrition 

 is usually affected by simple absorption of the juices of the 

 host. The exact physiological reason for the disappearance 

 of the gut is not obvious. Further, the method by which 

 such parasites are protected from the action of the ferments 

 of their hosts is not clear. The reason is perhaps in part 

 the thickness of the cuticle, which is composed of substances 

 not amenable to ferment action. Again, Frenzel claims to 

 have found an anti-enzyme in Gregarines, which neutralises 

 the action of the host's intestinal juices. The problem is 

 analogous to that suggested by the fact that the cells of 

 the gut escape during life the action of its juices, by which 

 they are often attacked after death. Frenzel, indeed, com- 

 pares a Gregarine to an absorbing intestinal cell. 



Digestion in the Invertebrata. 



In the Ccelentera, ferments have been extracted from the 

 bodies of jellyfish and sea anemones. In some cases a 

 tryptic ferment was extracted from the reproductive organs, 

 a peptic from the tentacles and mesenteries. The secretion 



