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MANUAL OF ELEMENTARY ZOOLOGY 



vesiculse seminales. The latter are large sacs, formed by 

 the walling-off of parts of the ccelom, which enclose the 

 testes. Each consists of a median part and lateral horns. 

 The anterior vesicula seminalis, in segment io, has four 

 lateral horns, two in front and two behind, which push out 

 the septa and bulge into the ninth and eleventh segments. 

 The posterior vesicula seminalis, in segment n, has only 

 two such horns, which project 

 into the twelfth segment. Each 

 sperm-mother-cell forms by multi- 

 ple fission, in the course of which 

 the usual reduction division takes 

 place, a mulberry-like mass (Fig. 

 406) consisting of little cells 

 attached to a central mass of 

 residual protoplasm known as the 

 cytophore, by which they are 

 nourished. The little cells be- 

 come pear-shaped, with the broad 

 ends on the cytophore, gradually 

 increase in length, and change 

 their shape till the mulberry has 

 become a tuft of threads, each 

 thread being a spermatozoon with 

 a very slender head. Finally the 

 spermatozoa break loose. In the 

 median part of each vesicula semi- 

 nalis, directly behind the testes, is 

 a pair of large ciliated funnels with 

 folded walls, known as sperm 

 -One of the ovaries rosettes. These funnels lead into 

 the vasa deferentia, of which the 

 two on each side join and pass 

 backwards to open on segment 15. The cilia of the 

 rosettes draw the ripe sperm into the ducts. 



Pairing takes place at any time from spring to autumn 

 in warm, damp weather. Two worms stretch themselves 

 out of their burrows and place their ventral sides together 

 with the heads pointing in opposite directions, their bodies 

 being held together by a substance secreted from the 

 clitella. Sperm is passed from the vas deferens of each 



Fig. 145.- 



of an earthworm. 



