152 



ORGANS AND ORGAN SYSTEMS 



pair at least may be hidden by the overlying seminal vesicles 

 or organs for the manufacture of spermatozoa. At the period 

 of maturity these receptacles are usually filled with mature 

 spermatozoa which have been formed in another worm. When 

 the eggs are mature these spermatozoa are squeezed out and 

 fertilization takes place on the outside of the body. 



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FIG. 63. Diagram of the reproductive system of the earthworm showing the 

 central chamber of the seminal vesicles and the internal position of the testes. 



The spermatozoa-forming organs are more complicated con- 

 sisting essentially of three pairs of closed sacs called the 

 seminal vesicles united in the median line and enclosing the 

 sperm mother cells derived from the testes. Strictly speaking 

 there is but one sac for the cavities of the seminal vesicles are 

 all in open communication, the walls of the sac being drawn out 

 in three pairs of lobes. The testes are small and difficult to find 

 in the mature worm for the dorsal wall of the vesicle sac must 

 first be removed. One pair are situated on the posterior side 



