MEANS AND METHODS OF REPRODUCTION 



157 



state. Thus the vas deferens of vertebrates is a modified 

 excretory duct (Wolffian duct) and some of the tubules of the 

 testis are modified uriniferous tubules. 



Reproduction in the Earthworm. — The manner in which repro- 

 duction takes place in the earthworm illustrates the process in a 

 hermaphroditic animal, somewhat higher in the scale than Hydra. 

 The testes of the earthworm consist of two pairs of organs lying 



N 



10 



11 



13 



14 



15 



Fig. 102. — Diagram of the reproductive organs of earthworm, n, nerve cord; 

 o, ovary; od, oviduct; sd, sperm duct (vas deferens); sr, sperm receptacles; sv, 

 sperm vesicle, with part of the roof removed to show the testes, T, and the funnels 

 of the sperm ducts. 



in the body cavity, one pair on the posterior side of the anterior 

 wall of segment 10, and the other pair in a similar position on the 

 anterior wall of segment 11. These same segments each contain 

 a pair of ciliated funnel-shaped openings of the sperm ducts or 

 vasa deferentia. Two ducts of the same side unite posteriorly 

 to form a single duct or vas deferens, which opens externally on 

 segment 15. Three pairs of seminal vesicles, communicating 

 with each other, extend over segments 9 to 15. Those belonging 

 to segments 10 and 11 enclose the testes and the mouths of the 

 sperm ducts. When the sperm cells reach a certain stage of 



