x PHYLUM ANNULATA 471 



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. 

 Impregnation takes place externally in nearly all. 



In the Oligochaeta 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 vesiculae 

 seminales to undergo development into mature sperms. The 

 vesiculce seminales are comparatively large sacs, which vary in 

 number and arrangement in the different genera. One or two 

 median sperm-reservoirs, formed by the coalescence of pairs of 

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

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

 either two or four ciliated funnels, 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. In connection with the terminal part of each vas 

 deferens in many Oligochaets is a gland known as the prostate or 

 spermiducal gland. Near the aperture of the vas deferens in many 

 Earthworms are special setae, the penial setce. 



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 be 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 

 coelome. Receptacula seminis are present. 



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 

 invagination (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. The micromeres spread over the megameres very 

 much as in the Polychseta. A pair of mesoderm cells early appears, 

 and by their division forms the mesoderm bands. No free larval 

 stage similar to the trochophore occurs in any of the Oligochseta, 



