482 



ZOOLOGY 



SECT. 



sometimes they surround one of the principal blood-vessels, 

 sometimes they are situated laterally', in the bases of the para- 

 podia. The sperms frequently undergo the final stages of their 

 development after they have become detached from the testes, 

 while floating in the coelomic fluid, and the same sometimes 

 holds good of the ova. Both sperms and ova appear to reach the 

 exterior, in the majoiity of cases, through the " segmental organs," 

 which may become modified and enlarged at the breeding season, 

 though in some forms it is stated that the reproductive cells escape 



ss v&ni.uesa 



Fig. 378.— Diagram to illustrate the development of a gonad from the peritoneal (ccelomic) 

 epithelium in one of the Polychajta. jnr;/. peritoneal membrane ; re'pv. gl. gonad (repro- 

 ductive organ) ; vent. vess. venhal vessel. (After E. IMeyer.) 



through temporary or permanent openings in the body-wall. Im- 

 pregnation takes place externally in nearly all. 



In the Oligochfeta the reproductive organs are confined to a 

 certain limited region of the body. There are either, as in the 

 Earthworms, two pairs of testes, or a single pair, as in the aquatic 

 forms. The testes are small, and frequently become reduced to 

 mere vestiges in the adult animal, having mainly become broken 

 up into sperm-mother-cells, which in some way reach the vesiculas 

 seminales to undergo development into mature sperms. The 

 vesiculm seminales are comparatively large sacs, which vary in 

 number and arrangement in the different genera. One or two 

 median spe.rm-sacs, formed by the coalescence of pairs of vesiculse, 

 may be present. In the same segments as the testes, and 

 opening into the sperm -sacs when the latter are developed, are 



