MOLL USCA^ONTOGENY 



255 



the auricle are distinct. An ectodermal depression on tlie foot forms the operculum. 

 The mantle cavity which lies on the right side, and into which the rectum opens, 

 now also stretches to the left on the anterior and dorsal side of the sharply 

 demarcated visceral dome. The gill appears in the form of a protuberance on the 



Fio. 211.— Development of Paludina vivipara (aftor v. Erlanger). A, An embryo in which 

 the first rudiment of the gill has appeared. B, A nearly mature embryo. Both are seen from the 

 left side. Lettering as in Pig. 210. In addition, 17a, Auricle ; 176, ventricle ; 18, nephridium ; 19, 

 rectum ; 20, rudiment of the radular sac ; 21, rudiment of the gill ; 22, osphradium (Spengel's 

 organ) ; 23, rudiment of the genital duct ; 24, rudiment of the gonad ; 25, operculum. 



inner surface of the mantle cavity, and the osphradium at the left of the gill as an 

 ectodermal protuberance. 



Fig. 211 B finally shows us an embryo in which the mantle cavity has assumed 

 the anterior position on the visceral dome. The ctenidium and osphradium have 

 developed further. The velum is very much reduced and can only be seen in 

 sections. This stage is important on account of the appearance of the rudiment of 

 the genital organs, which is identical in the two sexes, A depression of the (meso- 



