x PHYLUM ANNLLATA 483 



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 ms defer ens, opening at the male genital 

 aperture. In connection with the terminal part of the vas 

 delerens in many OligochaBts is a gland known as the prostate or 

 spcrmiducal gland. Near the aperture of the vas deferens in 

 many Earthworms are special seta?, the penial setcv. 



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 

 ccelome. 



Development. The OligochaBta 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 (Lumbricns rubcllus) ; 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 Luwibricus 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 Polychaeta. A pair of mesoderm cells early appear, 

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

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

 but the stage intervening between the completion of the gastrula 

 and the commencement of the segmentation of the mesoderm 

 bands corresponds to the trochophore in essential respects ; and in 

 some forms there is recognisable a feebly developed circlet of 

 cilia comparable to the prototroch, and in some a pair of head- 

 nephridia. 



Impregnation and the development of the embryo takes place 

 externally in all the ChaBtopoda, with a very few exceptions in 

 which development occurs in the coelome or in the interior 

 of a dilated segmental organ. In the Polycha3ta, in the great 

 majority of cases, fertilisation takes place by the sperms coming 

 in contact with the ova when both have become discharged, 

 and the development of the embryos goes on while they are 

 floating freely in the sea. There are a few cases in which the 

 impregnated ova are received into a sort of brood-pouch and 

 there pass through at least the earlier stages of their development. 



i I 2 



