DIGESTIVE SYSTEM OF THE EARTHWORM 137 



pairs of peculiar calciferous glands. These glands consist of 

 numerous flat and broad pockets of tissue radially arranged on 

 the oesophagus as axis. The flattened pockets are enclosed in a 

 muscular sheath and lie in a blood-filled sinus, while between 

 the pockets are collections of lime. These " glands" are not 

 true glandular diverticula of the oesophagus but are mesodermal 

 in origin and are merely the walls of the blood vessels. The cells 

 of the pockets take crystals of calcium carbonate from the blood 

 and secrete them in a milky fluid into the oesophagus. (See 

 Combault, Harrington, etc.) 



The function of the glands is not entirely clear although several 

 assumptions have been made which are more or less well grounded. 

 Claparede and Darwin believed that the milky fluid is an excretion 

 of the great quantity of lime which is contained in fallen leaves 

 and accumulates in the blood after digestion of the leaves. Harrington, 

 on the other hand, found that secretion of lime from the glands 

 diminishes if the worms are fed with calcium carbonate, but increases 

 if fed with acidified food and he accepts a second hypothesis of 

 Darwin's, viz., that the lime plays a role in digestion. This role is to 

 neutralize the humus acids contained in decomposing vegetation, and 

 to prepare a suitable alkaline medium for the action of tryptic 

 ferments. 



A third view advanced by Combault is that the calciferous glands 

 form a sort of internal breathing organ for removing C02 from the 

 blood, combining it with calcium and excreting it as lime into the 

 oesophagus. This view, which is also supported by experimental 

 evidence, does not exclude the possibility of a digestive function, but 

 if true, adds the further function of preventing a surplus of CO 2 in 

 the blood. 



The Crop'and Gizzard. In the i4th segment the digestive 

 tract enlarges to form a thin-walled expansion called the crop, 

 extending from the i4th to the i6th somites. No special func- 

 tion is attributed to this organ, but it opens directly into a 

 thick- walled gizzard provided with powerful circular muscles. 

 The contraction of these muscles acting on the contained food 

 material mixed with gravel, results in the trituration of the 

 solid materials and prepares them for digestion in the 

 stomach intestine. 



The Stomach Intestine. This, the most important organ of 



