782 COMPARATIVE PHYSIOLOGY. 



absence of an enzyme-containing secretion. The blood con- 

 tained in its pouched gut is simply absorbed by the wall.s. 

 The similarity of this method of nutrition to the purely 

 parasitic one found in Cestodes and Trematodes, has been 

 advanced as a physiological 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, etc. a ferment capable of acting upon proteids 

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

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

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

 only active in neutral or alkaline solutions. 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. 



In the Echinoderms we find that in star-fishes tryptic, 

 peptic, and diastatic ferments are all found ; the voluminous 

 caeca serve as reservoirs for the secretion. In the Holo- 

 thurians no digestive glands have been, as yet, found in 

 connection with the gut, nor can any ferment be extracted 

 from its walls. The contents of the gut are, however, mixed 

 with a peptic ferment ; this can also be extracted from 

 extra-intestinal parts of the body, so that ferment-secreting 

 glands must exist. A similar diffuseness in the occurrence 

 of ferments is very common among the Echinoderma. It is 

 therefore asserted that digestion must go on in various parts 



