No. I.] THE EMBRYOLOGY OF CREPIDULA. 145 



paarig sind, wird keine Schwierigkeit machen, da die Entwick- 

 lungsgeschichte von Umbrella fur eine urspriinglich paarige 

 Anlage spricht. Auch die Lage des Organs am hinteren Kor- 

 perende braucht noch kein Grund dagegen zu sein. Durch die 

 Untersuchungen von McMurrich wissen wir, dass die ausseren 

 Exkretionszellen der Prosobranchier ebenfalls bald etwas weiter 

 vorn, bald etwas weiter hinten sich befinden." This difference 

 in position seems to me, however, to be a very considerable 

 one. In all prosobranchs these cells lie close behind the velum, 

 while in Umbrella they are removed from that structure by 

 almost the whole diameter of the embryo. Further, the fact 

 that they sink into the interior in Umbrella would indicate 

 that they are different from the excretory cells of proso- 

 branchs. 



Rabl ('79) believes that the so-called " primitive excretory 

 cells " have no excretory function at all, but are merely a part 

 of the velum. The fact, however, that in very many forms 

 they are found to contain granules or crystals which have been 

 seen to be extruded from the cells lends support to Bobretzky's 

 ('77) idea that these cells really have an excretory function, and 

 the fact that they are completely cast off in Crepidula would 

 still more strongly support that view. That they are any 

 portion of the velum, as has been maintained by Rabl and 

 McMurrich, seems to me to be distinctly negatived by their 

 general position behind that organ and their complete separa- 

 tion from it. 



Bobretzky ('77) says that the ectoderm passes unbroken 

 beneath these cells, while McMurrich ('86) believes that the 

 excretory cells form a part of the layer of ectoderm covering 

 the embryo, and that therefore they are not underlaid by ecto- 

 derm. In Crepidula, as we have seen, these excretory cells at 

 first form a part of the general ectodermal layer, and are not 

 underlaid by ectoderm ; in later stages, however, the ectoderm 

 forms a layer beneath them very nearly if not quite complete. 

 If the conditions which prevail in Crepidula are general, it is 

 probable that McMurrich based his conclusions on the study 

 of younger stages, while Bobretzky was guided by the study of 

 older ones. 



