i] STRUCTURAL AND SYSTEMATIC 5 



known as the clitellum. Each of the 218 segments 

 of the body except the first, and possibly one or two 

 at the hinder end, is furnished with eight minute 

 projecting bristles, the setae ; these are disposed in 

 pairs and all lie upon the ventral aspect of the worm. 

 The movement of these by special muscles aids in 

 locomotion. 



An examination with even a hand lens shows a 

 number of external pores w r hich are important. 

 Anteriorly there is the mouth which is overhung 

 by the prostomium referred to above. At the 

 extreme hind end and surrounded by the last 

 segment of the body is the vent. Along the middle 

 line of the back are a series of pores, one just at the 

 very anterior edge of each segment, through which, 

 when the worm is dried and then slightly pressed, 

 liquid is seen to be ejected. These are called the 

 dorsal pores and they belong one to each segment 

 with the exception of the first seven, or in some 

 cases more, segments. In front of one or other of 

 the pair of setae which is situated most laterally, 

 i.e. furthest from the ventral median line, is an orifice 

 on each side in all but the first one or two segments 

 of the body. These paired pores are the external 

 outlets of the excretory organs frequently termed on 

 account of their regular repetition with the segments 

 'segmental organs,' but more conveniently to be 

 named nephridia. In the clitellar region and in fact 



