492 EDWTN S. GOODRICH. 



secutive peritoneal funnels, and by the invagination of the 

 epidermis at the genital pore to form an atrium and penis. 



We may now sum up the main characteristics of the nephridia 

 and genital ducts in this group, -which has been treated at 

 length owing to its great importance (figs. 5, 6, 7, and 25). 



The nephridia of the Oligochsetes are probably of epiblastic 

 origin. They develop from large cells ("funnel-cells'*), 

 arranged metaraerically outside and between each pair of 

 somites. They pass through a more or less disguised prone- 

 phridial stage (comparable to that permanently retained in 

 flatworms, &c.); in the first (most forms), and sometimes in 

 the trunk segments (Chaetogaster) they never develop beyond 

 that stage. In the other segments the nephridia grow towards, 

 and open into, the coelom by means of a funnel formed from the 

 original "funnel-cell.'*^ 



The genital ducts, on the other hand, are peritoneal funnels 

 of undoubted mesoblastic origin, which grow outwards from the 

 metameric genital follicles to open to the exterior. They thus 

 have no connection with the nephridia, and differ from them 

 entirely in their development. 



HiRUDINEA. 



So closely do the coelom, genital ducts, and nephridia of the 

 Leeches agree in their development with those of the Oligo- 

 chaetes, that their history need only be rapidly sketched. 



Biirger (16 and 19) has carefully traced, in several forms, 

 the origin of the whole nephridium proper (funnel and canal) 

 from a large " funnel-cell," which comes to lie in the hinder 

 wall of each coelomic follicle. Just as in the previous group of 

 worms, this cell buds off a row of cells behind which constitute 

 the canal; the "funnel-cell** then divides up into a ring of 

 small cells, which form the funnel of the adult nephridium 

 (figs. 8 and 9). This organ remains closed in some forms, such 

 as Hirudo, but opens into the coelom in others, such as Nephelis. 



' The branched so-called plectonephric condition of the nephridia in certain 

 earthworms has recently been shown to arise by the secondary subdivision of 

 originally paired nephridia (Vejdovsky, 102; A.. G. Bourne, 13). 



