88 Descriptive Zoology. 



time, except after a heavy rain, it is pretty good evidence 

 that they are diseased. Of ten % in such cases it is found 

 that they have been parasitized by a fly. Nearly every 

 one must have noticed in the morning the fresh excrement, 

 at the mouths of their burrows. These coiled "castings," 

 as they are called, are the residue of digestion, and as the 

 amount of nourishment in the soil is not great, and since 

 the worm must do considerable excavating, the amount of 

 the "castings" is necessarily considerable. In dry wea- 

 ther the worms dig deep, and may be several feet from the 

 surface. But when the ground is fairly moist they often 

 remain during the day near the surface, with one end near 

 the end of the hole. In winter they hibernate below the 

 reach of frost. 



Form of the Earthworm. The end that usually goes 

 foremost is the anterior end, and the hinder end is the 

 posterior end. When crawling on the ground the surface 

 on which the worm rests is the ventral surface, and the 

 surface uppermost is the dorsal surface. If the earthworm 

 were split lengthwise in the middle line by a vertical plane, 

 the right and left halves would be counterparts of each 

 other, that is, the earthworm is bilaterally symmetrical. 

 The earthworm is approximately cylindric, the anterior 

 end being more pointed, and the posterior end somewhat 

 flattened, especially on the ventral surface. The division of 

 the body into rings, or segments, is very evident. Toward 

 the anterior end is a region of about six segments in which 

 the sides and dorsal portions of the segments are swollen 

 and more or less fused together, forming a wide girdle 

 called the clitellum, the function of which is to secrete 

 the capsule in which the eggs are laid. 



General Plan of Structure. As just noted, the body of the 

 earthworm is cylindric. At the anterior end is the mouth and 



