THE EARTHWORM 



169 



upon + thele = nipple), ep, which secrete the digestive fluids 

 and also aid in the absorption of the food. Just outside of 

 these is a layer of blood vessels, v. A third layer consists 

 of circular muscle fibers extending around the intestine, cm, 

 and outside of this is a layer of longitudinal muscles, Im. A 

 fifth layer on the outside consists of a thick coat of cells known 

 as chlorogogen cells, c. These cover the intestine with a 

 thick layer on its outer surface and also form the substance 

 of the typhlosole, which, as 

 shown in Figure 81, lies 

 within the cavity of the in- 

 testine. The function of the 

 chlorogogen cells is not 

 known, though it is probable 

 that they have something 

 to do with the absorption FIG. 83. MAGNIFIED VIEW OF A 

 of food and possibly have a SECTION OF THE ALIMENTARY CANAL 

 function of secretion. On 

 either side of the oesophagus 

 in the tenth, eleventh, and 

 twelfth segments are three 

 pairs of white bodies known as calciferous glands (Lat. calx 

 = lime -\-ferre = to bear), producing a lime secretion which 

 is poured into the intestine. Its function is probably to reduce 

 the acidity of the food, although very little is known about 

 these glands or their uses. 



The Nervous System. The microscopic study of the nerv- 

 ous system of the earthworm, as well as of all higher animals, 

 has shown that while there are several kinds of cells in ijb, 

 the chief ones, and probably the only ones possessing nervous 

 functions, are large cells called neurons. 



Neurons. A single neuron of the earthworm is shown in 

 Figure 84 A. It has a rather irregular rounded body, with a 

 prominent nucleus, and from it arises a long process, much 

 longer than appears in the figure. Side branches of this proc- 



c, chlorogogen cells; 



cm, circular muscles; 



ep, epithelium, lining the canal; 



Im, longitudinal muscles; 



v, blood vessels. 



(Modified from Sedgwick and Wilson.) 



