ON THE C(ELOM, GENITAL DUOTS, AND NEPHRIDIA. 481 



follicleswere larger, Gun da segmentata could be called a truly 

 segmented animal.^ The inner ends of the genital ducts are 

 formed as outgrowths from the genital follicles : '' Der Oviduct 

 ist bei Gunda segmentata, wie bei Planaria torva anfangs ein 

 solider Zellenstrang. Zweifellos ensteht er durch Wucherung aus 

 dem soliden ovarium selbst, ahnlich wie die Samenleiter Aus- 

 wiichse der Hoden sind" (observations confirmed in his later 

 work, 70). 2 The oviduct becomes hollow and ciliated, and grows 

 backwards to the genital pore; the vasa efferentia fuse to form 

 the main sperm-duct. 



To sum up, then. In the Planarians the excretory organs are a 

 pair of pronephridia, probably derived from the epiblast; the 

 gonads arise from a mass of cells in the mesoblast, which may 

 become hollowed out into a genital follicle (coelomic sac) from 

 the wall of which arise the genital cells. The follicle grows out 

 to form the genital duct (peritoneal funnel), which joins an 

 epiblastic invagination at the genital pore (fig. 1).^ 



' E. Meyer believes (81) that the ancestor of the Annelids possessed a pair 

 of long genital follicles, and that metamerism was brought about by their being 

 broken up at intervals, chiefly to facilitate its serpentine motion ; each portion 

 would then have acquired its own duct to the exterior. It seems to me more 

 probable that the metameric arrangement of the genital follicles is more 

 directly due to that "tendency" to repetition by a sort of budding, which is 

 seen in the case of the gonads, the penes (Anonymus), and even the pharynx 

 (Phagocata ; Woodworth, 112) amongst the Planarians, and again, perhaps 

 amongst the Mollusca (for a full discussion see Bateson, 3). 



2 Jijima's account of the development of the sperm-ducts differs somewhat 

 from that of Lang, but he derives both from the mesoderm (60). 



^ The spaces contained in the connective tissue or parenchyma have been 

 sometimes compared with the coelom ; these spaces seem rather to represent 

 the vascular system of the higher Coelomata. I need not treat here of the 

 homology of the vascular system, which is probably of quite separate origin 

 from the coelom. Professor Ray Lankester's view, that the blood-system is 

 simply a liquefaction, as it were, of the mesoblast, seems to me to agree per- 

 fectly with the facts. Moreover, the theory held by many authors that it is 

 directly derived from the blastoccel appears to be quite untenable. As Pro- 

 fessor Lankester has pointed out to me, if this were the case we should 

 expect to find the blood-spaces best developed amongst the Diploblastica ; 

 now it is just in the (adult) Coelenterates that it is entirely absent. Indeed 

 we may say that the blood-space, or heemoccel, does not appear in phylogeny 



