268 GENERAL BIOLOGY 



through the dorsal pores, it has likewise been suggested that they may 

 have some excretory function. 



Three pairs of calciferous glands (L\>f\<? C AX**^ & ) one P a ^ r 

 in each of the somites from ten to twelve are found at the sides of the 

 oesophagus. The first pair are pouches pushed out from the alimentary 

 canal which open directly into the oesophagus. The other two pairs are 

 swellings of the oesophageal wall. They have a number of small cavities 

 which open directly through the epithelium into the oesophagus in 

 somite fifteen. 



One writer thinks these glands manufacture carbonate of lime which 

 is then secreted into the alimentary tract to neutralize the acid foods, 

 while another suggests that the primary function of the glands is merely 

 to excrete calcareous matter derived from leaves on which the animal 

 feeds. This opinion he bases on the fact that such matter accumulates 

 in leaf-tissue and remains in the leaf when it falls. The worms taking 

 in large quantities of calcareous matter but having no' shell or bone, 

 there is no use for it, and so "some special excretory apparatus seems 

 necessary." This latter opinion does not oppose the one given imme- 

 diately preceding it. But the gizzard and intestinal content of worms 

 is, as a rule, acid, so this would seem to oppose both of the above ideas. 

 However, this acidification "may be the result of fermentations occur- 

 ring in the later stages of digestion." 



It will thus be seen, that many things must be considered before 

 one can speak on subjects such as these with any degree of authority 

 and positiveness. 



As stated, the earthworm feeds on decaying leaves and animal mat- 

 ter. This food is sucked into the buccal cavity. 



Here it receives a secretion from the pharyngeal glands, after which 

 it passes through the oesophagus to the crop to be stored temporarily. 

 Secretions from the calciferous glands in the oespohageal walls neutral- 

 ize the acids. The gizzard is a grinding organ in which the food is 

 broken up into minute fragments by being squeezed and rolled about. 

 Then, too, solid particles, such as rough pebbles, which are frequently 

 swallowed, may aid in the grinding process. The food then passes to 

 the intestine, where most of the digestion and absorption takes place. 



Digestion in the earthworm is very similar to that of higher ani- 

 mals. The digestive fluids act upon proteins, carbohydrates, and fats; 

 in them are special compounds called ferments or enzymes, which break 

 up complex molecules without themselves becoming changed chemically. 

 The three most important enzymes are (1) trypsin ( ), 



which dissolves protein; (2) diastase ( ), which 



breaks up molecules of carbohydrates 1 ', and (3) steapsin ( ), 



which acts upon fats. These three enzymes are probably in the digestive 

 fluids of the earthworm. The proteins are changed into peptones, the 



