EARTHWORMS, LEECHES, AND SEAWORMS 83 



nerve cord already described. This explains the fact that 

 the sense of touch is well developed and extends over the 

 whole surface of the body. It is thought that the sense of 

 taste is located in the mouth and pharynx. Darwin has 

 shown that the sense of sight, feeble though it is, is con- 

 fined chiefly to the anterior end of the body, although the 

 posterior end is sensitive to light. It is certain that the 

 earthworm perceives the difference between darkness and 

 light for it withdraws to its burrow during the daytime, and 

 when a bright light is flashed upon the anterior end, it con- 

 tracts. It has no eyes, and we have no reason to suppose 

 that it is capable of forming pictures of objects. Neither 

 is there evidence to show any sense of hearing. 



Its food and how eaten. — The food of the earthworm 

 comprises both animal and vegetable matter, the latter 

 consisting largely of bits of leaves. Besides these articles 

 of food it swallows large quantities of earth that pass slowly 

 through the aUmentary canal, during which passage the 

 nutritive portions contained in it are absorbed and digested. 

 This earth is usually obtained at a considerable depth, but 

 the waste parts of it thrown off by the aUmentary canal are 

 deposited at the surface of the ground and form the so- 

 called "castings" about the mouths of their burrows. 



The pharynx is muscular and contractile. Moreover, 

 muscles run from the outside of the walls of the pharynx 

 to the inside of the body walls. These expand the pharynx, 

 and if the mouth has been previously applied to any soUd 

 object, such as a leaf or pebble, the pharynx acts upon it 

 Uke a suction pump and draws it into the alimentary 

 canal if desired. 



Regeneration of lost parts. — Earthworms are remarkable 

 for their abiUty to withstand injury and to regenerate lost 



