ON THE CCELOM, GENITAL DUCTS, AND NEPHRIDIA. 485 



stammt und dass diejenigen Beobachter, welche ihr aus dem 

 Ektoderm enstehen lassen, entweder nur den ausfiihrenden 

 Theil der Niere beriicksichtigt haben oder, was noch haufiger 

 geschicht, die beiden Abschnitte nicht in ihren Zusammenhaug 

 erkannten : Der ausflihrende Theil wird namlich von Allen, 

 mit Ausnahme von Rabl^ aus einern Theil der Mantelhohle 

 abgeleitet " (32). On the left side, to which the genital 

 function is restricted, the gonad develops from the wall of the 

 ccelom ; then, together with the rudimentary left peritoneal 

 funnel, it becomes constricted off from the main division of 

 the coelom (the pericardium), forming a small genital sac. 

 Prom the wall of this sac the genital duct grows out, and 

 joins an epidermal invagination, like the peritoneal funnel of 

 the right side. Doubtless the genital duct is really the left 

 peritoneal funnel, as v. Erlanger suggests : " Ich konnte .... 

 feststellen, dass die Anlage der Genitaldriise in der urspriing- 

 liche linken Halfte des Perikards ensteht, und zwar ungefahr 

 da, wo sich die rudimentare linke Niere zuriickgebildet hat. 

 Eben so ensteht auch die Anlage des Ausflihrganges an der 

 Stelle, wo der rudimentare Ausfiihrgang der linken Niere sich 

 befand, und scheint einfach aus diesem hervorzugehen." These 

 observations have been fully confirmed in the case of Bythinia 

 (33), and the conclusions, as we shall see shortly, are supported 

 by the comparative anatomy of the MoUusca in general 

 (fig. 22). 



Ziegler has described the development of Cyclas cornea, 

 which in some respects is less modified, the adult being 

 symmetrical (116). Here also the coelom (pericardium) arises 

 as a right and left follicle. The two cavities enlarge, surround 

 and fuse below the gut. On either side the organ of Bojanus 

 develops as a peritoneal funnel, which meets an epidermal 

 invagination (fig. 22). 



In the Mollusca, then, we find at an early stage a pair of 

 true nephridia (Urniere, head-kidneys), possibly of epiblastic 

 origin. The coelom develops as two cavities in the mesoblast, 

 genital follicles, from the walls of which grow out two 

 peritoneal funnels, the organs of excretion and carriers of the 



