140 ORGANS AND ORGAN SYSTEMS 



form three 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.) 



t 



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 may be 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 CO2 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, it indicates the 

 further function of preventing a surplus of COo 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 1 6th somites. No special func- 

 tion, apart from storage, 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 food materials and prepares them for 

 digestion in the stomach intestine. 



The Stomach Intestine .--This, the most important organ of 

 the alimentary system, begins at about the i8th somite and runs 



