364 OLIGOCHAETA 



perfectly normal in structure and in position. So also are the 

 oviducts ; but both are enclosed in sacs which communicate in 

 rather an elaborate fashion. Each ovisac is somewhat rounded 

 in form, and the two communicate by a narrow tube ; from the 

 ovisac also arises another narrow tube, which soon dilates into 

 a chamber lying in the thirteenth segment ; this contains the 

 mouth of the oviduct and is continuous with the egg-sac ; the 

 latter is quite normal in position. Beyond the egg-sacs the two 

 tubes unite round the intestine and open into a large median sac, 

 which contains sperm and may be called the spermathecal sac 

 (1). There is, however, a true spermatheca, single and median. 

 This opens on to the exterior in the middle of the thirteenth 

 segment, but lies chiefly in the right-hand sac behind the ovarian 

 portion of the same. I never found this spermatheca to con- 

 tain sperm. Dr. Rosa inferred on anatomical grounds, and I 

 have been able to prove developmentally (in Libyodrilus), that 

 these sacs which involve the ovaries and oviducts, and which also 

 contain sperm, are derivatives of the septa ; that in fact the spaces 

 which they enclose are coelomic. In some Eudrilids these sacs 

 are the only " spermathecae " ; in others, as in Hyperiodrilus, 

 there are in addition blind pouches lying within them which 

 must be regarded as true spermathecae ; these are smaller in some 

 than in others. In fact there are various transitions in the entire 

 replacement of true spermathecae apparently homologous with 

 those of other earthworms by pouches which are derived from 

 the septa, and which are therefore of an entirely different morpho- 

 logical significance ; here is an excellent case of the substitution 

 of organs, analogous to the replacement of the primitive noto- 

 chord of the Vertebrate by the vertebral column. 



So far as is known, all the Oligochaeta deposit their eggs in 

 special chitinous cases, the cocoons. They share this peculiarity 

 with the Hirudinea. The cocoons have long been known, but 

 were originally mistaken for the eggs themselves. The cocoons 

 contain several eggs and a variable quantity of albumen for the 

 nutrition of the growing embryos. In the majority of earth- 

 worms they are more or less oval with projections at the two 

 ends, and are of a brownish colour. In others the tint is rather 

 to be described as green. The genera Criodrilus and Spargano- 

 jt/iilvs have a cocoon .which is greatly elongated. These struc- 

 tures seem to be undoubtedly formed by the clitellum, the earlier 



