THE EARTHWORM AND ANNELIDS IN GENERAL 165 



portion being known as the anterior, the hinder portion as the 

 posterior end. The principal axis of the body is antero-posterior, 

 and, since the chief organs both external and internal lie half on 

 one side and half on the other side of a median plane, the worm is 

 said to be bilaterally symmetrical. 



A noticeable feature of the earthworm is its division into a large 

 number of similar rings by grooves extending around the body at 

 short intervals. These rings are termed 

 segments, somites, or metameres, and the 

 body is said to be segmented or to have 

 a metameric structure. The earthworm 

 and many of its near relatives differ in 

 this respect from the unsegmented flat 

 worms and round worms, such as 

 Planaria and Ascaris. The somites are 

 not exactly alike. The anterior lobe 

 extending above and beyond the mouth, 

 and backward on the dorsal surface in- 

 tersecting the first segment is not a true 

 somite; it is known as the prostomium. 

 It has been found that not only external 

 structures, but also internal organs, bear 

 a constant relation to the segments; 

 for this reason the somites have been numbered, beginning with 

 the one just back of the prostomium. In mature worms the 

 six or seven somites, XXXI or XXXII to XXXVII, are swollen 

 on their dorsal and lateral surfaces, producing a saddle-shaped 

 enlargement known as the clitellum, of use during reproduction. 



If an apparently smooth earthworm is drawn through the 

 fingers from its anterior end toward the tail, it feels rough to the 

 touch. This is caused by small /-shaped chitinous bristles, called 

 setce (Fig. 84), four pairs of which extend outward from epidermal 

 sacs in every somite except the first and last. Each pair may be 

 moved by retractor and protractor muscles. The arrangement 

 of the setae is shown in the tigh^-nand half of the section in Figure 



V \J 



FIG. 84. Two setae of Lum- 

 bricus, highly magni- 

 fied. (From Parker 

 and Haswell.) 



