254 PHYLUM ANNELIDA 



(2) The two oviducts open internally on the anterior face 

 of the septum between 13-14, and externally on the ventral 

 surface of segment 14. Into the wide ciliated internal 

 mouths, which lie opposite the ovaries, the ripe eggs pass. 



(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 of this one. They are white 

 globular sacs, opening in the grooves between segments 

 9-10 and lo-ii, and probably, like the genital ducts, arise 

 from modified nephridia. According to some, these sper- 

 mathecae 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 with the head 

 of the one towards the tail of the other, with about 40 

 segments overlapping. The spermatozoa of the one pass 

 along a groove into the receptacula of the other. 



When the eggs of an earthworm are hberated, they are surrounded 

 by a gelatinous sheath secreted b'y the saddle. As this is peeled off 

 towards the head, spermatophores are also enclosed. When free, the 

 ends of the sheath close, and a lemon-shaped cocoon results. 



Development. — 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 earth- 

 worm of the dungheap (L. fcetidus) makes this a habit, induced probably 

 by the warmth of its environment. 



Of the many ova in the cocoon of L. terrestris, only one comes to 

 maturity, while in L. fcetidus a few, and in L. communis two may do 

 so. But in the last species the two embryos are often twins formed 

 from one ovum, separation taking place at the gastrula stage. 



The whole process of growth, until leaving the egg, lasts from two 

 to three weeks, the time varying, however, with the temperature. 



The ovum is surrounded by a vitelUne membrane, and is laden with 

 yolk granules. Segmentation is slightly unequal (Fig. 137 (i)), and 



