THE OKYELOPMENT OF FASC1OLAKIA. 153 



might be homologous with the primitive kidneys of Lyiu incus and 

 Planorbis, the external excretory organs " Urnierenzellen " of 

 these forms could not be homologized with the primitive kidneys 

 of Pahtdina. 



Thus the significant fact was brought out that there are two 

 kinds of " Urnieren," the one, since proved to be of mesodermal 

 origin (Palndina, Planorbis], opening to the exterior by a pore, 

 the others, collections of modified ectoderm cells, situated either 

 on the postoral row of the velum (I.yjnnceiis, Planorbis], or be- 

 hind the velum (Orpidnla, Fn/gitr]. The confusion due to the 

 attempts to homologize the external kidneys of prosobranchs 

 with the primitive kidneys of pulmonates, and the converse prop- 

 osition of tracing an homology between the external kidneys of 

 pulmonates and the primitive or head kidneys of prosobranchs, 

 was probably greatly increased because all these structures were 

 called by some German writers "Urnieren." In this connection 

 it is interesting that Korschelt and Heider ('93) in copying Bob- 

 retsky's figures for their text-book changed the labelling so that 

 " Un " is replaced by " Ex." 



Rabl ('79) came to the conclusion that the external kidneys of 

 Planorbis were a part of the velum, and that they have nothing to 

 do with the " head kidneys." Sarasin ('82) describes a most in- 

 timate connection between the velum and the external kidneys 

 in Bithynia tentaculata. In fact he described the two under one 

 name, the " Ansae." In speaking of their homologies he says : 

 " Nach den Erfahrungen von Bobretsky, Biitschli und mir, liegt 

 auf jeder seite der Prosobranchier Embryonen, ein Haufchen 

 grosser Ectodermzellen, das bei Paludina und Bithynia mit Wim- 

 feroffnung nach aussen miindet. Nach Biitschli und Fol finden 

 sich diesselben bei Planorbis. 1st dies richtig, so haben die Siiss- 

 wasser Pulmonaten zwei Organ paare, die als Urniere zu deuten 

 sind ein vorderes und ein hinteres Paar. Hat Rabl recht dass 

 die von Biitschli zuerst gefundengrossen Zellen bei Planorbis und 

 Lymncens zum Velum gehoren, so sind wahrsheinlich die von 

 Biitschli und mir bei Pahtdina und Bithynia gefundenen Organe 

 den hinternen Urnieren der Siisswasser Pulmonaten homolog." 

 Sarasin's position, except that it admits the ectodermal origin of the 

 primitive kidneys in Paludina, is in the present state of our knowl- 



