THE PHYSIOLOGY OF NUTRITION. 739 



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 intra-cellular ; in Hydra, both intra- and extra- 

 cellular digestion seem to occur. 



Among the higher worms, Hirudo is distinguished by the 

 absence of an enzyme containing secretion. The blood con- 

 tained in its pouched gut is simply absorbed by the walls. 

 The similarity of this method of nutrition to the purely 

 parasitic one found in Cestodes and Trematodes, has been 

 advanced as an additional reason for associating the leeches 

 with flat worms rather than with the Chaetopoda. The habit 

 of feeding on the blood of other animals may, however, 

 have led to some of the leech's peculiarities. 



In most of the other Annelida Aphrodite, Arenicola, 

 Lumbricus, &c. a ferment capable of acting upon proteids 

 has been found. It is closely allied to the tryptic ferment 

 of Vertebrates, but is not identical in all its chemical reac- 

 tions. It has been termed iso-trypsin, and like trypsin it is 

 only active in neutral or alkaline solution. It appears to be 

 confined to the Annelida. The intestinal " caeca " found in 

 Aphrodite and others are not absorptive areas, but merely 

 reservoirs of secretion. They are rendered necessary by the 

 fact that the gland cells are constantly active, and not merely, 

 as in Vertebrates, stimulated to action by the presence of 

 food in the intestine. The process is therefore closely 

 analogous to the secretion of bile by the vertebrate liver, 

 where the liver cells are constantly active, and the gall 

 bladder, like the caeca of worms, serves as a store chamber. 

 But as the bile is probably not to any extent a digestive 

 fluid, and as the true digestive glands of Vertebrates are 

 not constantly active, the conclusion is suggested that the 

 constant activity of the cells in the worm is a primitive 

 condition. In most Annelida a diastatic ferment also 

 occurs, which possesses as usual the power of converting 

 starch into sugar. 



Turning to the Echinoderms, we find that in star fishes 

 tryptic, peptic, and diastatic ferments are all found. The 



