CH. l] EARTHWORMS AND LEECHES 17 



worms. The spermatozoa flow backwards from the male 

 aperture in a longitudinal groove on each side to the 

 receptacula (spermathecse) of the other worm, the grooves 

 of the two animals together forming a temporary tube. 

 Hence only one worm can emit spermatozoa at any given 

 time, otherwise there would be opposing currents. The 

 worms are so placed (Lumbricus terrestris) that the 9th 

 segment of each is opposite the 32nd (1st clitellar) of 

 its mate, then the thickened clitellum forms a barrier, 

 past which no flow of seminal fluid can take place. In 

 this position the modified and grooved genital setae of the 

 26th segment come opposite the small aperture in the 

 loth segment, and are probably used to hold the lips of 

 this opening apart during sexual congress. The long 

 genital setse in the "tubercula pubertatis" of the 

 clitellum, and of segments 10 to 15, are probably used, 

 the former to liberate the cocoon from its seat of 

 origin, and the latter series to hold the cocoon off the 

 ventral surface in the region of the oviducal openings 

 and those of the spermathecse, and thus allow ova and 

 spermatophores to pass into the cocoon as it passes forwards. 

 These specialised setae replace those of ordinary form as 

 the worm reaches maturity. The eggs do not pass 

 out of the oviduct till near the end of the act of mating. 

 Each of the two worms forms a cocoon, and slips out 

 of it backwards, passing it forward over its head. The 

 cocoon being elastic closes its two open ends as soon as the 

 body of the worm is withdrawn, and becomes more or less 

 lemon-shaped, its bulging centre being occupied by about 

 4 eggs, spermatozoa and albuminous material produced by 

 L. 2 



