196 SEGMENTED WORMS OR ANNELIDA. 



(3) The egg sac or receptaculum ovorum, near the internal 

 mouth of each oviduct, is a posterior diverticulum of the 

 septum between segments 13-14. Within it a few mature 

 ova are stored. 



(c) Two pairs of receptacula seminis or spermathecae 

 receive spermatozoa from another earthworm, and liberate 

 them to fertilise the eggs^f this one. They are white 

 globular sacs, opening in the grooves between segments 9-10 

 and 10- 1 1. According to some, these spermathecae not 

 only receive and store spermatozoa, but make them into 

 packets or spermatophores. Others say that the glands of 

 the clitellum make these packets. At any rate minute 

 thread-like packets of spermatozoa are formed, and a pair 

 of them may often be seen adhering to the skin of the 

 earthworm about the saddle region. 



When two worms unite sexually they lie apposed in 

 opposite directions, the head of the one towards the tail of 

 the other. What happens is that spermatozoa of the one 

 pass into the receptacula of the other. 



When the eggs of an earthworm are liberated they are 

 surrounded by a sheath of gelatinous stuff said by some to 

 be secreted by the saddle. As this is peeled off towards 

 the head a spermatophore is also enclosed. 



Development of the Earthworm. 



Many cocoons are made about the same time, and each 

 contains numerous ova, and also packets of sperms, so that 

 fertilisation takes place outside the body. These cocoons 

 are buried in the earth a few inches below the surface. 

 They measure about a quarter of an inch in length. 



The favourite time for egg laying is during the spring 

 and summer, though it may be continued throughout the 

 whole year. The earthworm of the dung heap (L. fatidus) 

 makes this a habit, induced probably by the warmth of its 

 habitat. 



Of the many ova of the earthworm L. terrestris, only 

 one comes to maturity, while of L. fatidus a few, and of 

 L. communis two may do so. But in the last species the 

 two embryos are often twins formed from one ovum, separa- 

 tion taking place at the gastrula stage. 



