744 COMPARATIVE PHYSIOLOGY. 



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 protoplasm ; but 

 that, as we ascend in the scale of being, these ferments are 

 more and more utilised in the digestive processes, 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 effected 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. 



In the Ccelentera, ferments have been extracted from the 

 bodies of jelly-fish and sea-anemones. In some cases a 

 tryptic ferment was extracted from the reproductive organs, 

 a peptic from the tentacles and mesenteries. The secretion 

 of ferment is thus not confined to the digestive region, and, 

 according to Krukenberg, the ferments are not employed for 

 the digestion of food outside the formative cells. In his 

 experiments he found that solution and absorption of food 

 particles only took place when the particles were in actual 

 contact with the digestive region. In Sponges, digestion is 

 purely intracellular; in Hydra, both intra- and extracel- 

 lular digestion seem to occur. 



Among the higher worms, Hirudo is distinguished by the 



