EARTHWORM 85 



region of the animal is somewhat darker. Certain epidermal 

 cells secrete mucus which passes out through openings in the cuticle 

 and covers the body. Sensory cells are also present in the epi- 

 dermis. The next layer of the body wall consists of circular 

 muscle tissue. This tissue is formed from a great many long, 

 spindle-shaped muscle cells which are so arranged that when they 

 contract the diameter of the body is decreased. Beneath the 

 circular muscle layer is a thick layer of longitudinal muscle 

 tissue in which the cells are arranged with their long axes parallel 

 to the long axis of the body so that when they contract they cause 

 a decrease in the length of the animal. The innermost layer of the 

 body wall is known as the coelomic epithelium. It consists of 

 flattened epithelial cells which form the lining of the coelom. 



1. Nutritive System 



Beginning with the mouth opening at the extreme anterior end 

 of the body, the alimentary canal continues as a straight tube 

 throughout the entire length of the animal and finally ends at the 

 anal opening situated at the extreme posterior end. The following 

 regions may be noted. (W. f. 60.) 



The pharynx extends from the mouth through segment vi. It 

 is a heavy-walled, spindle-shaped structure and is attached to the 

 body wall by a great many fine muscle fibers. 



The esophagus begins at the posterior end of the pharynx in 

 segment vii and continues posteriorly as a narrow, straight 

 tube to segment xiv, where it connects with the crop and gizzard. 

 Paired swellings on the sides of the esophagus in segments x-xiv 

 constitute the calciferous glands. 



The crop and gizzard constitute sac-like enlargements of the 

 alimentary canal lying between segments xiv and xix. Although 

 of about the same size and shape, these two organs differ consider- 

 ably in structure. The crop is thin-walled and elastic and serves 

 for the temporary storage of food, whereas the gizzard has heavy, 

 muscular walls, and is adapted for grinding the food. 



The intestine has its beginning at about segment xx and con- 

 tinues as a straight, tubular structure to the posterior end of the 

 body, where it ends at the anal opening. In each segment there 

 is a slight out-pouching of the intestinal wall. 



The structural features of the intestine as revealed by the micro- 

 scopic study of a transverse section may now be described. The 



