REPRODUCTION 



1G7 



at the same time. Cross-fertilization is assured in this case by the 

 arrangement of the generative organs and by the method of mating. In 

 mating, the bodies of two worms are closely applied by their ventral 

 surfaces, the heads pointing in opposite directions and the thickened band 

 or clitellum of each worm approximately opposite segments 7 to 12 of the 



Fig. 135. — Copulation of earthworms. (Courtesy of General Biological Supply House.) 



other worm (Fig. 135). In this position each worm secretes a slime tube 

 (Fig. 136) which sheathes its body. Spermatozoa are discharged into 

 the space between the slime tube and the body of the worm, are carried 

 backward within the slime tube by the muscular contractions of the body, 

 and finally are picked up by the seminal receptacles of the other member 



Fig. 136.- 



-Slime tube and cocoon of earthworm: above, in process of formation; below, 

 after slipping off the worm. (After Foot.) 



of the pair. A cocoon is secreted around each worm, and eggs are laid 

 in it. The cocoon with the eggs in it is then slipped off over the head 

 end, along with the slime tube, and spermatozoa are discharged into it 

 as it passes the seminal receptacles (see page 168). Fertilization occurs 

 in the cocoon. 



