ANNULATA. 



227 



when mature find their way into the water where fertilization 

 takes place. The organs are much more complicated in the 

 hermaphrodite worms. The spermatozoa are produced in 

 the testes, are passed into the seminal vesicles where they 

 are matured, and at the time of copulation escape to the 

 exterior by the vasa deferentia, to be deposited in the 

 sperm sacs or reccptacula seminis of another worm. From 

 this place, any time after copulation, the sperm is brought into 

 contact with the ova as they pass from the ovary, where they 

 are produced, to the egg-sac or to the exterior; or the sperm 

 and ova may be brought together after both have escaped from 

 the body. It is believed that in some instances at least the 

 genital ducts are modified nephridia. 



267. Reproduction and Development. Sexual repro- 

 duction is universal. As we have seen, copulation may occur 

 or the elements may come together in the water. In the 

 Oligochaeta and leeches the fertilized ova, or the ova together 

 with masses of spermatozoa, are enclosed in a cocoon of 

 secreted material and within this case the young worm is de- 

 veloped. In the Polychseta the larva undergoes its develop- 



FIG. 103. Two stages in the development of Nereis. A, 8-celled stage; B, i6-celled 

 stage, both viewed from the active or ectodermal pole, mi. 1 , mi. 2 , and mi. 3 , the first, 

 second and third sets of micromeres separated from ma., the macromeres; s 1 , first 

 somatoblast, one of the second group of four cells to be budded from the macromeres; 

 s 2 , second somatoblast, one of the third group, which gives rise to the mesoderm. The 

 micromeres are ectodermal and the macromeres produce the entoderm. (After West- 

 inghausen.) 



