306 THE EARTHWORM AND OTHER ANNULATA 



beneath the cHtellum and opposite the openings of the seminal 

 receptacles of each worm, by an expansion of the ventro-lateral 

 edges of the bodies and a hollowing out of the median ventral 

 regions. The mucus secreted on the surface stiffens and thus the 

 two individuals become enclosed in a common tube extending from 

 segment fifteen to a region posterior to the clitellum. The worms 

 are now ready for the exchange of spermatozoa. In mature 

 specimens there are two parallel lines on either side of the body 

 and extending from the openings of the vasa deferentia to the 

 clitellum (c/. Fig. 137 A). These are the superficial indications of 

 muscular differentiations of the body wall which form, during the 

 stages of sexual union that follow, channels for the passage of 

 seminal fluid from one worm to the other, in the following manner. 

 A ridge with a groove on its dorsal and ventral edges is formed 

 on either side of each individual in the way shown by Fig. 147 B. 

 The ventral grooves are mostly hidden by the close apposition of 

 the bodies, but the dorsal ones are exposed (Fig. 147 A). It can 

 be seen that the dorsal grooves extend from the openings of the 

 vasa deferentia of one worm to the cavity beneath the clitellum 

 and opposite the seminal receptacles of the other worm. Since 

 the groove is roofed over by the mucus that encloses the individ- 

 uals it is actually a closed tube extending from the opening 

 of the vas deferens of one worm to the openings of the seminal 

 receptacles of the other. Small masses of the seminal fluid 

 containing sperm may now be seen, through the transparent 

 mucus, being ejected at regular intervals from the vasa deferentia. 

 As fast as one of these is emitted it is enclosed in a pit-like depres- 

 sion of the groove about the length of a single segment, which 

 then passes posteriorly as a wave-like depression, carrying the 

 mass of sperm slowly backward to the cavity beneath the clitellum. 

 These masses appear as a row of white rod-shaped objects travehng 

 along the line of the groove. When the seminal fluid of one 

 individual has thus accumulated opposite the openings of the 

 seminal receptacles of the other, it is forced into the latter, appar- 

 ently by contractions of the cHtellum, aided perhaps by a suck- 

 ing action of the receptacle. In this manner the mutual exchange 

 of spermatozoa occurs apparently without mixture of the seminal 

 fluid derived from the two individuals. The worms now 

 wrench themselves apart and are ready for egg laying at a later 

 time. 



