ON THE OCELOM, GENITAL DUCTS, AND NEPHRIDIA. 493 



Meanwhile the lumen of the canal in the posterior chain of 

 cells becomes hollowed out. The peripheral end of the canal 

 fuses with an invagination of the epidermis, the vesicle, by 

 means of which it opens to the exterior. The first origin of 

 the " funnel-cells/' from which the nephridia are formed, has 

 not been traced in detail; it seems quite probable that they are 

 derived from the nephric rows described by Whitman (107) in 

 Clepsine. (They are possibly the large cells mistaken by 

 Whitman [106] for the forecasts of the testes, as suggested by 

 Bergh.) 



As in the Oligochseta and Polycheeta, so in the Hirudinea 

 the nephridia of the anterior segments develop precociously in 

 the larva. Bergh (7) has traced their origin as outgrowths from 

 the epiblastic cell-rows. In Aulastoma there are four pairs, 

 which never develop beyond the pronephridial stage, i.e. do not 

 open internally. Bergh was also unable to find external 

 openings (compare the nephridia of Capitella, 30 and 31). 



The ccelom develops in a normal manner as a hollowing out 

 of the paired metameric blocks of mesoblast. Most of these 

 ccelomic or genital follicles are fertile: an anterior pair develop 

 the ovaries on the peritoneal wall; several posterior pairs 

 develop the testes. That part of the ccelom which surrounds 

 the gonads generally becomes partially separated off as a peri- 

 gonadial coelom (Bourne, 12a ; Burger, 16). That the genital 

 ducts are peritoneal funnels is shown by their development, 

 although it seems to be somewhat modified. The oviducts 

 arise from the ccplomic epithelium surrounding the ovary, and 

 fuse with the two ends of a forked, but median invagination of 

 the epidermis (fig. 9) (Biirger, 19). The vasa efferentia are 

 similarly formed from the testes (fig. 28); they grow outwards 

 and forwards, fusing with those in front. The most anterior 

 join the ends of a median forked invagination of the epidermis 

 (Nussbaum, 84; Burger, 19). The complete genital ducts thus 

 closely resemble those of some Planarians (Gunda), and differ 

 essentially from those of the earthworm only in the number of 

 peritoneal funnels which contribute to their formation (compare 

 also the Vertebrates). 



