MORPHOGENESE VON FUSUS 



877 



SUMMARY 



1. The authors have studied the embryonic development and the postem- 

 : i bryonic period during the first three weeks of Fusus, particularly the gut and the 

 -ii larval organs as larval nephridia, vacuolized cells of the skin, vélum, cephalic 



vesicle, larval heart, closing mechanism of the œsophagus, etc. 



2. The great amount of the protolecith is concentrated in four macromeres 

 i which show no further segmentation; the final absorption takes place during the 



postembryonic phase. 



3. The early absorption of capsular fluid by the pneveliger influences strongly 

 . the whole development, but nécessitâtes the formation of a specialized ingestion 

 J stage. It retards temporarely the development of the palliovisceral complex as 

 J well as of the foregut and the nervous System. 



4. The transitory swallowing mechanism of the œsophagus and the store sac 

 ... (nutritive sac) with its large vacuolized cells as apex of the left diverticulum of 



the hepatic gland form specialized adaptations for the résorption, respectively 

 J the storage of the capsular fluid. 



5. The différent types of cells in the hepatic gland change during ontogenesis 

 m the following way (see tab. V). The store cells of the left diverticulum are 



, j replaced by résorption cells transformed postembryonally into secretionresorption 

 . :ells (SRC). During postembryonic stages, the polynuclear sécrétion cells (fer- 

 . nentative cells) are confined by the outspreading SRC to the apex of the right 

 iigestive sac. 



6. The crawling stage at hatching ressembles externaly the adult. Its ganglia 

 . : ire relatively large compared with the proportions of the adult. The larval 

 rjI iigestive tract contains considérable food reserves; therefore its post-embryonic 



ransformation lasts more than a month. 



7. The pluriphased organogenesis together with the spécial adaptations for 

 :he ingestion of the perivitelline fluid and for the intracapsular life places Fusus 



i — with Pomatias and Cymba — as the most evolved species amongst the Proso- 

 i )ranchs without food-eggs. — A comparison with the Pulmonates shows identical 

 dations between nutritive sac and hepatic gland for many forms. 



LITERATUR 



ynkel, W. E. 1936. Prosobranehia. Die Tierwelt der Nord- und Ostsee. IXb x , Leipzig. 

 ' îloch, S. 1938. Beitrag zur Kenntnis der Ontogenèse von Susswasserpulmonaten mit 

 besonderer Beriicksichtigung der Mitteldarmdruse . Rev. Suiss Zool. 45 : 

 157-220. 



