88 H. J. FLEUKE. 



of schematic cross-sections of tbe mantle cavities of various 

 forms^ showing the rectum and oviduct or right kidney duct 

 cut through, and the relative position of the left kidney or 

 the Mouotocard kidney opening. 



It will be seen that the openings of right and left kidneys 

 iu Diotocards have the same positions, with reference to one 

 another, as the genital and excretory apertures in Monoto- 

 cards. The relation of these openings to the rectum, on the 

 other hand, varies to some extent in different forms. 



The most striking feature of these diagrams (11 — 15) is 

 the migration of these openings to the animal's right side, 

 and we also notice the disappearance of the right ctenidium 

 and the folding over to the left side of the originally right 

 leaf of the other ctenidium. 



In the primitive Gastropods, as in Cephalopods, the in- 

 coming streams of water entered the gill cavity on either 

 side and bathed the gills, after which they made their way out 

 along the median line of the cavity, taking away the excreta 

 from the openings on this line (fig. 11). Later on, the left 

 kidney degenerated, being perhaps partly pressed out of 

 existence, and the right kidney became the sole functional 

 excretory organ. This process, already begun in Haliotis, 

 was correlated with the disappearance of the ctenidium from 

 the right side, and to the shifting of anus, renal, and 

 reproductive openings to this side. The respiratory stream in 

 such forms (figs. 12 — 15) would now come iu along the left 

 side and go out past the anus, etc., along the right, in- 

 current and excurrent streams being thus freed from mutual 

 interference. This more perfect separation, Professor Davis 

 thinks, was the advantage which led to natural selection of 

 variations along the lines of the changes just mentioned. This 

 clockwise shifting of apertures and ducts has iu some cases 

 been continued so far that one or more have become situated 

 along the extreme edge or even, in some cases, on the Hoor 

 of the mantle cavity, so that their original relations appear, 

 in horizontal plan, to have been reversed. Paludina shows a 

 further modilication, for here the I'cual ouening is situated 



