AN EARTHWORM 239 



in size, which occur in rows on the ventral surface of every 

 somite except a few at the anterior end. The openings of 

 the body are the mouth at the anterior end just beneath the 

 prostomium (lip), the anus at the posterior end, and various 

 openings on the ventral surface, which are connected with 

 internal organs to be described later. An earthworm has 

 no eyes, yet it knows of the existence of light, and shuns 

 intense light ; it has no ears, but it becomes aware of the 

 approach of an enemy by the jar communicated through 

 the soil. In a mature earthworm a thick band, consisting 

 of the thickened body wall, is visible at about one third the 

 length of the animal from the anterior end. This is called 

 the clitellum. 



All the many species of earthworms agree very closely in 

 general structure and arrangement of the organs of the 

 body. However, the following discussion is of the single 

 species Lumbricus terrestris. Locations of the various organs 

 will not agree exactly for any other species. Besides the 

 more technical points of difference Lumbricus terrestris may 

 be recognized as the species that has the tail very much 

 flattened. 



The internal anatomy of an earthworm seems, on refer- 

 ence to Fig. 128, to be very complicated, but each system 

 of organs has well-defined uses. 



Digestion. The first division of the digestive system of the 

 earthworm is the mouth (Fig. 128), followed by the pharynx, 

 the latter having a very thick muscular wall. The pharynx 

 can be protruded slightly or retracted, and the cavity en- 

 larged by strands of muscle fibers which extend to the body 

 wall. Food, consisting of particles of leaves, animal tissues, 

 and even soil, is drawn into the pharynx by the sucking 

 action which takes place when the cavity of the pharynx is 

 enlarged. The food passes directly through the esophagus. 

 Before the ingested (swallowed) food reaches the crop the 



