n6 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



food is brought into condition suitable for absorption, 

 is unable to exercise its activity unless the food on 

 which it acts is alkaline in reaction ; in other words, 

 its activity is arrested in an acid solution. Now, the 

 process of the decay of leaves is accompanied by the 

 formation of several acids, which must necessarily be 

 neutralised before the digestive fluids can act on the 



Fig. 57. Transverse Section of Earthworm to show the Position and 



Eelations of the Intestine. 



a, Cuticle ; 6, hypodermis ; c, layer of circular muscles : d, layer of longitu- 

 dinal muscles; i. enteric cavity; m, "green layer"; n, dorsal vessel; 

 o, " liver." (After Clapardde.) 



ingested leaves ; this neutralisation appears to be 

 effected by the calcareous concretions on which the 

 so-called humus acids readily act ; the result of their 

 union is an alkaline liquid. 



Below the calciferous glands the esophagus widens 

 out into a crop, and this is succeeded by a gizzard, 

 which is provided with powerful transverse muscles, 

 and ordinarily contains, as has been already stated, 

 small stones and grains of sand ; by the powerful con- 

 traction of its muscular walls and by the aid of these 



