THE EARTHWORM 



305 



Reproduction and Life Cycle. — In the earthworm, as in many 

 hermaphroditic animals, sexual union consists in the temporary 

 pairing of two individuals with a mutual exchange of spermatozoa. 

 In L, terrestris, at the time of this union two individuals come 

 together with their heads pointing in opposite directions and the 

 ventral surfaces of their bodies in contact (Fig. 147 A), so that seg- 

 ments nine, ten, and eleven of each individual are opposite the 



A 



B 



Fig. 147. — Sexual union and egg-laying in earthworm. 



A, two worms encased in mucus secreted by skin glands, as they appear during passage 

 of spermatozoa along seminal grooves as shown by arrows. B, transverse section showing 

 encasing mucus and four seminal grooves {g) on each individual. T, formation of egg 

 capsule and its passage anteriorly (a to h) until it is slipped off as the fully formed capsule 

 (f/. Fig. 143 C). 



clitellum, segments thirty-one to thirty-seven, of the other; while 

 the ventral elevations about the openings of the vasa deferentia on 

 the fifteenth segment of one worm he opposite, or perhaps a little 

 anterior to, the twenty-sixth segment of the other indi\adual. 

 The posterior ends of the worms usually retain their holds upon 

 the burrow throughout the entire process. As the adjustment 

 of the individuals to one another becomes more complete, mucus 

 flows copiously from the clitellum and from other skin glands in 

 this anterior region of the body. Also, a depression is formed 



