1883,] NATURAL SCIENCES OF PHILADELPHIA. 239 



place is wliei'e the rectum, which is on its way perpendicularly^ 

 through this part of the animal to the gill, comes in contact with 

 the tube. The anterior part of this canal I call the prterectal, 

 and the posterior portion the postrectal. 



This little canal has nearly the same calibre throughout ; the walls 

 of the praereetal part are composed of cylindrical epithelial cells, 

 which lie on a fine tunica propria, and on the free ends of which 

 are found cilia. The cilia are longest at the pericardial opening 

 of this tube. The lumen of the postrectal part is nearly the same 

 as that of the pr.ierectal part ; the walls of the former, however, 

 are somewhat thicker. 



The internal surface of the exeretor}- organ is also ciliated, and 

 consists of a layer of cylinder epithelium. In the walls are 

 found those concretions so characteristic of the gastropod kid- 

 ney. These concretions are not found in the walls all over the 

 kidney, but seem confined to a certain part. It is my opinion 

 that the concretions are identical to those small granulations 

 referred to by Moquin-Tandon (see p. 237) in the mucus of this 

 region. 



The sacular portion of the kidney does not pass graduall}" into 

 the tubular portion, but at a sharp angle, as is seen in the diagram 

 (PI. X, fig. 10), where a little blind sac is formed (PI. X, fig. 

 10 z). The diameter of this part of the sacular portion is 

 0*2 mm. 



The second part of the kidney, or the tubular portion, is 

 much longer than the pericardial or sacular portion, but has a 

 much smaller diameter than the latter, and is convoluted. At 

 the beginning it runs parallel with the inferior border of the 

 mantle, and bending at r (PL X, fig. 10) it returns on its course; 

 at c' (fig. 10), it makes another bend and passes for a short distance 

 forward again ; then forming a slight curve it passes to its most 

 inferior position, and then running parallel with the lower border 

 of t-lie mantle it opens at os, at a position about opposite the pos- 

 terior part of the gill. In the diagram (fig. 10) I have represented 

 the convolutions as if the}^ were all in one plane; this is, however, 

 not the case, as in a horizontal section we often see two convo- 

 lutions. 



In A. lacustris the kidney has essentially the same form, lying 

 in the right mantle, save that the folds of the sacular portion are 

 not so marked. 



