The structure of the female organs differs considerably in detail in the different genera. But _Hyperiodrilus_ may be taken as an instance of a genus in which these organs are as complicated as they are anywhere. The ovaries (Fig. 193, 4) are {364}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 contain 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 morphological significance; here is an excellent case of the substitution of organs, analogous to the replacement of the primitive notochord of the Vertebrate by the vertebral column.