196 GENERAL ZOOLOGY 



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

 are themouth'&t 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. The 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 the 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 ditellum. 



PHYSIOLOGICAL PROCESSES 



The internal anatomy of the earthworm seems, on reference 

 to Fig. 100, to be very complicated, but we shall find that 

 each system of organs has well-defined uses. From the study 

 of the internal organs represented here we shall learn some- 

 thing of the uses of internal organs in general; and we 

 shall endeavor to understand more fully the nature of physi- 

 ological processes, which have been briefly referred to in the 

 chapter on the locust. We have already become familiar with 

 the organs in which these processes take place under the 

 names digestive system, circulatory system, respiratory sys- 

 tem, excretory system, and the like. 



Digestion. The first division of the digestive system of the 

 earthworm is the mouth (Fig. 100, 6), followed by the pharynx 

 (Fig. 100, 7), the latter having a very thick muscular wall. 

 The pharynx can be protruded slightly or retracted, and the 

 cavity enlarged by strands of muscle-fibers which extend to 

 the body-wall (Fig. 100, 9). 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 



