442 ZOOLOGY SECT. 



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 majority of cases, through the nephridia, which 

 may become modified and enlarged at the breeding season, though 

 in some forms it is stated that the reproductive cells escape 

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

 pregnation takes place externally in nearly all. 



In the Oligochseta 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 vesiculse 

 seminales to undergo development into mature sperms. The 

 vesicular seminales are comparatively large sacs, which vary in 

 number and arrangement in the different genera. One or two 

 median sperm-sacs, formed by the coalescence of pairs of vesiculaB, 

 may be present or absent. In the same segments as the testes, 

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

 either two or four ciliated funnels (mesonephridia), according to 

 the number of the testes, leading into efferent ducts. All the 

 four ducts, when four are present, may remain distinct, or the 

 two ducts of each side may open into a common atrium, or they 

 may unite to form a common elongated vas deferens, opening at the 

 male genital aperture. 



There are never more than two ovaries, which, like the testes, 

 are of very small size. The ova may become mature in the ovary, 

 or groups of cells may become detached from the latter and one 

 cell in each group ripen into an ovum. A receptaculum ovorum 

 occasionally receives the ova after they leave the ovary. There 

 are two oviducts, which open by funnel-shaped apertures into the 

 ccelome. 



Development. The Oligochseta deposit the eggs in cocoons, 

 either buried in the earth or attached to water-plants. The 

 cocoon contains, in addition to a number of fertilised ova, a quan- 

 tity of an albuminous fluid which serves as nourishment to the 

 developing embryos. Segmentation is always unequal. In the forms 

 in which food-yolk is scanty there is a process of embolic in- 

 vagination (Lumbricus rubellus): in the others (Tubifex, &c.) the 

 process is of the epibolic type. In the former case a blastula and 

 an invaginate gastrula are formed in the way already described 

 in the case of the Earthworm. In Lumbricus trapezoides the 

 gastrula divides into two, each half subsequently giving rise to 

 an embryo. In the latter the micromeres spread over the mega- 

 meres very much as in the PolychaBta. A pair of mesoderm cells 

 early appear, and by their division form the mesoderm bands. No 

 free larval stage similar to the Trochosphere occurs in any of the 



