156 ANNELIDA [CH. 



Every earthworm has grown up from an egg which has been 

 fertilised by a spermatozoon. As the earthworm is hermaphrodite, 

 that is to say, contains both male and female organs, it might be 

 thought that the spermatozoa- ot an individual would fertilise its 

 own ova, but this is not the case. Cross fertilisation or the 

 fertilisation of the ova of one individual by the spermatozoa of 

 another is the rule in Nature, and the earthworm is no exception 

 to the rule. The method by which the spermatozoa reach the ova 

 is not clear in all its details, but it is something like this. The 

 cells which are to form the spermatozoa break off from the testes 

 and whilst lying in the fluid contents of the vesicula seminalis 

 they divide and the products of the division or spermatozoa de- 

 velop each a long vibratile tail by whose aid they swim actively 

 about. Two earthworms then approach each other and place their 

 ventral surfaces in contact. The heads of the pair are turned in 

 opposite directions. Since the ventral surfaces are slightly grooved 

 a sort of tube is formed by the opposition of the two worms, and 

 the pair are bound together by a mantle of slime secreted by the 

 goblet cells. The spermatozoa of one of the pair, which acts as 

 male in this embrace, pass down the sperm funnels and vasa 

 deferentia into the tube formed by the adpressed ventral surfaces 

 of the worms. The spermathecae of the one which acts as female 

 are filled from this tube. The earthworms then separate, one 

 carrying away the spermatozoa of the other 



The spermathecae in which the earthworm stores up the 

 spermatozoa received from another individual are pockets of the 

 skin (1, Fig. 64). They belong, strictly speaking, to the female 

 reproductive system. Seen from the interior of the animal, they 

 appear as four small white spherical bodies, lying one pair near the 

 hind end of segment nine, and the other pair near the hind end of 

 segment ten, and each pair opens by a very short neck or duct on 

 the grooves between segments nine and ten and ten and eleven, 

 just inside the outer pair of chaetae. It is through these ducts 

 that the spermatozoa from another worm enter. 



Earthworms lay their eggs in cocoons, which at one time were 

 mistaken for the eggs themselves. These cocoons are usually brown 

 and horny and vary in size in different species of earthworm ; some 

 are about as large as rape seed, others almost equal in bulk to a 

 small grain of wheat. They are formed from the secretions of the 

 peculiar ectoderm cells found in the clitellum and at first have 

 a ring-like shape. The secretions harden when in contact with the 

 air. The animal begins to wriggle out of the band, which at first 



