EGG-LA YING. 77 



of lateral vesicles. The two median vesicles thus formed eiivehji) 

 the testes and sperm-funnels uf their respective somites and hide 

 them from view. 



The sperm-cells leave the testis at a very early period and float freely 

 in the cavities of the seminal vesicles, where many stages of their develop- 

 ment may easily be observed. They are developed in balls known as 

 spermatospheves, each of which consists of a central solid mass of proto- 

 plasm surrounded by a single layer of sperm-cells. When mature the 

 spermatozoa separate from the central mass and are drawn into the fun- 

 nels of the sperm-ducts. The manner in which this action is controlled is 

 not understood. 



The seminal rece;ptacles are accessory organs of reproduction 

 in the shape of small rounded sacs or potiches, open to the out- 

 side only, at about the level of the upper row of set*. They 

 lie between the 9th and 10th, and lOtli and lltli somites (*./•, 

 Figs. 24 and 29), where their openings may be sought for (Fig. 

 21). Their function is explained under the head of co])ulati()n. 



Accessory glands. Besides all the structures so far described 

 there are many glands which play a part in tlie reproductive 

 functions. The setigerous glands from about the 7th to about 

 the 19th somite (sometimes fewer, sometimes none at all) are 

 often greatly enlarged, and form the glandular j^rominences men- 

 tioned at p. 46. They seem to be used as organs of adhesion 

 during copulation. The clitellum is tilled with gland-cells which 

 probably serve in j)art to secrete a nourishing tluid for the young 

 worms, and in part to provide a tough protecting membrane to 

 cover them. 



Copulation. Egg-laying^. Inasmuch as each individual earth- 

 worm produces both ova and spermatozoa, it might be supposed 

 that copulation, or the sexual union of two different individuals, 

 would not be necessary. This, however, is not the case. The 

 ova of one individual are invariably fertilized by the s])ermatozoa 

 of another individual after a process of copulation and exchange 

 of spermatozoa, as follows : During the night-time, and usually 

 in the spring, the worms leave their burrows and ])air. placing 

 themselves so that their heads point in opposite directions and 

 holding firmly together by the enlarged setigerous glands and the 

 thickened lower lateral margins of the clitellum. During this 

 act the seminal receptacles of each w<^rm are filled witli s]>erma- 

 tozoa from the sperm-ducts of the other, after which the wnrms 



