PHYLUM ANNELIDA 209 



crawling movements are brought about by " chain reflexes " 

 which pass down the body, each part setting off the one 

 behind. 



The earthworm possesses a great number of small sense 

 organs which, though slightly more abundant toward the 

 ends, are distributed rather uniformly in the skin through- 

 out the body. It is able to taste or smell with equal facility 

 at any point on the outer surface and therefore shows little 

 or no cephalization of the chemical sense. An earthworm 

 has no eyes, but is sensitive to light over its whole surface, 

 and in this respect also shows a diffuse condition. In each 

 segment of the body, sense organs are less abundant in the 

 middle and more numerous toward the anterior and posterior 

 ends. When the body is contracted, therefore, the ma- 

 jority of these organs are folded into the grooves between 

 the segments, and cannot be stimulated. It is possible that 

 the earthworm may rest by contracting the body to cover 

 most of the sense organs, just as a man closes his eyes when 

 asleep. 



Though the earthworm is structurally and physiologically 

 a chain of short units which work with more or less in- 

 dependence and in a rather mechanical way, it is able to 

 carry on some activities which require the correlation of all 

 parts of the body. Above the pharynx is a ganglion, or 

 " brain," and there are three giant nerve fibers which run 

 through the whole nervous system from end to end. When 

 the entire body wiggles, the activities of the short nervous 

 units are probably controlled and coordinated from the 

 brain through these long fibers. There is also one type of 

 sensation which is apparently localized at the anterior end. 

 If an earthworm is allowed to crawl about on a moist surface 

 of limited area, it withdraws when it comes to dry spots at 

 the edge, but if the prostomium (or upper lip) is removed 

 such reactions cease. The ability to discriminate between 

 wet and dry surfaces is, however, the only sense that is 

 known to be thus localized. 



Though the earthworm and its close relatives are rather 



