ANNELIDA : THE EARTHWORM, NEREIS 2iJ 



the hinder region is being drawn up. A wave of move- 

 ment of this kind passes down the body. The ventral setse 

 of the clitellum, of the twenty-sixth, and of the tenth to the 

 fifteenth segments are straighter and more slender than those 

 of other segments, which are stout and somewhat hooked. 

 This modification is in connection with the use of the setse 

 of the twenty-sixth segment during coition, and of the other 

 straight setse during the formation of the cocoon in which 

 the eggs are laid. 



A number of internal organs open separately upon the 



surface of the body. We have already men- 

 Openings. tioned the mouth and anus. The openings of 



the vasa deferentia are a pair of slits with 

 swollen lips found on the under side of the body in 

 segment 15. In front of them, in segment 14, are the two 

 small openings of the oviducts. The spermathecal pores 

 are two pairs of small, round 

 openings in the grooves 

 between segments 9-10 and 



io-ii at the level of the Fiq _o ne of the d'n 

 lateral seta;. The nephri- "'• setae ofMearthworm?"^ 

 diopores are openings which moved from the seta sac 



lead from the excretory and magnified, 



tubes or nephridia. They 



are found, as a pair of minute pores in front of the ventral 

 setse, in each segment except the first three and the last. 

 The dorsal pores are small, round openings on the mid- 

 dorsal line in the grooves between the segments. The first 

 is behind the eighth segment, and there is one in each 

 subsequent groove. They open into the body cavity, the 

 fluid in which oozes out through them and moistens the 

 surface of the body, mingling with the slime secreted by 

 the unicellular glands of the skin. As this fluid contains 

 cells which attack bacteria and other small parasites, it is 

 a valuable defence to the worm against such enemies, which 

 are numerous in the soil. 



The body of the worm may be said to consist of two 



tubes, one within the other. The inner tube is 

 Body-wan. the ^ the outer the bodv _ wa u Between the 



two lies the ccelom or body cavity, divided into compart- 

 ments by a series of septa, which reach from the gut to the 



