THE DEVELOl^MENT OF PALUDlNA VIVIPAEA. 99 



regard to tlie manner of development wliich differ from those 

 of von Eriauger in certain important points, and wliich bring 

 Paladina even more closely into line with other Molluscs in 

 respect of their coelom. 



According to von Erlanger, the pericardium^ wliile still 

 showing its two distinct chambers, forms two little evagina- 

 tious, one on each side, wliich are the rudiments of a pair of 

 kidneys. These, from tbe time of their first formation, lie 

 agninst the ectoderm, which very soon forms the inward duct- 

 like prolongations of the mantle cavity. Of these tlie right 

 one coalesces with the original right kidney and forms its 

 duct, while the left is arrested in its growth and the left 

 kidney disappears. At a later stage a new outgrowth of 

 the pericardium takes place in the same position as that 

 wliich formed the original left kidney, becomes nipped off 

 from the pericardial epithelium, and forms a little vesicle, 

 wliich is the rudiment of the gonad. At the same time 

 there is an ingrowth of the mantle cavity, which is pre- 

 sumably the arrested duct of the kidney that has disappeared. 

 This grows towards the gonad, and finally fuses with it to 

 form its duct. This account of the origin of the urino- 

 genital organs has since been confirmed by the more recent 

 researches of Tonniges (17).^ 



I have nothing to add to von Erlanger's description of the 

 early stages of development of the pericardium and kidneys, 

 and of their relation to the mantle cavity. The series of 

 somewhat oblique transverse sections of which one is repre- 

 sented in fig. 1 shows just such a condition as von Erlanger 

 describes. In this figure the pericardium is shown with its 

 two chambers still separated, the right being very much the 

 larger of the two ; the first rudiment of the heart, as appear- 

 ing at h. ; and just to the left of this is seen the original right 

 kidney (r. k.) with its lumen, hardly showing in this section, 

 communicating with the cavity of the pericardium. In a 



' 1 liavCj uufurtuiiately, not beeu able to obtaiu access to the orij^inal paper 

 by Toiuiiges in the ' S.B. Gcs. Bef. d. ges. Natuiw., Marburg,' fur 1899, and 

 have had to rely upon the abstract iu the ' Zool, Ceutralb.' 



