CH. I] EARTHWORMS AND LEECHES 7 



organic acids resulting from decay of vegetable substances 

 in the soil, and thus assists the digestive action of the 

 alkaline fluid which is poured over the leaves, and which 

 continues its action subsequent to the act of swallowing. 

 Nevertheless the contents of the gizzard and intestine are 

 as a rule acid, probably as a result of fermentations 

 occurring in the later stages of digestion. Harrington 1 

 finds that excess of acid in the food increases the amount 

 of lime produced by the glands. 



The crop serves as a temporary storage place in which 

 the digestive processes continue prior to the passage of the 

 food into the gizzard. Here the strong muscular walls and 

 horny lining, aided by small particles of stone that are 

 almost invariably to be found in this cavity, grind and 

 triturate the food so that the contents of the intestine are 

 always in a state of extremely fine division. Miss Green- 

 wood 2 has described retractile cilia upon the epithelium 

 of the intestine, and states that digestion is effected by 

 the secretion of unicellular glands over the whole of the 

 typhlosole and corresponding part of the intestine. The 

 typhlosole, which forms so conspicuous a fold along the 

 dorsal wall of the anterior portion of the intestine, is a 

 contrivance for increasing the area of the internal absorbing 

 surface without increasing the external bulk of the tube. 

 The cylindrical body of a worm does not permit of enlarg- 

 ing the absorptive surface by augmenting the length of 

 the intestine and throwing it into loops an arrangement 

 found in the convoluted intestines of most vertebrate 



1 Journ. Morph. vol. xv. suppl. 1900. 



2 Journ. Physiol. vol. xin. 



