OLIGOCHAETA. 507 



furrows) in such a way that the openings of the spermathecae are opposite 

 the clitellum of the other worm. The bodies become adherent, probably by 

 suction of the ventral surface of the clitella, with the assistance of a secretion 

 of the clitellum, and sperm is poured out of the openings of the vasa deferentia. 

 This is passed back along the ventral body of the worm in grooves which, can be 

 faintly discerned in sexually mature worms, leading back from the male genital 

 opening. These grooves end at the clitellum, and the transmission of the sperm 

 backwards is probably effected by the coordinated contraction of their sides. 

 However that may be, the sperm is transported backwards until it comes to 

 the ventral surface of the clitellar region ; here it is opposite the openings 

 of the spermathecae of the adherent worm. These appear to take it up by 

 a suctorial action, and so to become filled with the sperm of the other worm. 

 The use of spermatophores in such cases is unknown.* 



Asexual reproduction is found in some of the aquatic forms, and 

 the earthworms seem to have considerable power of reproducing 

 lost parts. 



The asexual reproduction consists of a process of fission combined 

 with gemmation. It takes place at certain periods of the year under 

 certain climatic conditions, and is exhibited by worms which are 

 immature in the fact that the generative organs are not developed. 

 In the Naididae (Nais, Cliaetog aster, etc.) a zone of fission is formed, 

 i.e. one of the somites enlarges and becomes divided into two parts : 

 the anterior part forms the anal segment of the anterior worm, and 

 the posterior gives rise to the head and anterior segments of the 

 posterior portion. A chain of zooids may be formed in this way, 

 the individuals of which eventually separate. In the earthworms 

 the anterior end, including the pharynx and cerebral ganglia, can 

 be reproduced after excision, and the same remark applies to the 

 hind end.f 



The Oligochaeta are distributed all over the world in fresh-water 

 and on the land. Some are amphibious, and a few species are 

 marine; but the majority are terrestrial, and are earthworms in the 

 -ordinary sense of the word. 



Earthworms are found in the surface soil of the earth, in which 

 they form burrows, coming to the surface to get rid of their faeces 

 (worm casts), and to procure food, and to copulate. Their food 

 consists of decaying vegetable substances, which they find in the 

 form of leaves and twigs on the surface and in the dark vegetable 

 surface soil, large quantities of which they swallow. The part which 

 earthworms play in modifying the surface soil is well known, and 

 has been fully treated by Darwin in his celebrated work on the 

 subject. Briefly their action may be summed up as follows : they 



* Andrews, American Naturalist, 1895. t Billow, Z. f. w. Z., 1883. 



