174 GUY C. ROBSON 



From the rectal sinus blood passes by rather irregular and 

 inconstant lacunar spaces to the roof of the paUial cavity, 

 and finally into more definite and constant lacunae at the base 

 of each gill-lamella. Thence it passes to the afferent vessels, 

 and from these through the blood-spaces in the lamellae to 

 the efferent vessels at the apex of the plates. The efferent 

 vessels of all the lamellae are united on the left-hand margin 

 of the gill by the pulmonary vein. On the left-hand side the 

 afferent vessels apparently lose themselves in lacunae. On 

 the rectal side the efferent vessels come to an end in the free 

 portion of each lamella. 



It is probable that blood is brought from the left-hand side 

 of the mantle cavity in a wide irregular lacunar system which 

 ultimately debouches into the same sub-lamellar spaces as the 

 blood from the rectal sinus. 



4. Eenal System (Text-figs. 5 and 7). 



The kidney hes between the body-wall, the pallial cavity, 

 and the pericardium, sending ramifications among some of the 

 other organs as in P . j e n k i n s i . It communicates with the 

 pallial cavity at its own anterior end by a ciliated aperture 

 furnished with sphincter and dilator muscles. In general 

 the kidney is a thin-walled cavity with a lining of special 

 secretory vacuolated cells as figured and described by Bregenzer 

 (6, p. 248). Anteriorly, however, there is a special area between 

 the body-wall and the pericardium characterized by a compact 

 stroma of connective tissue with blood cavities communicating 

 with the portal vein. This is the ' blood-gland ' of various 

 authors (cf . Simroth, 21, p. 564). It is present in B y t h i n e 1 1 a 

 but absent in Paludina, Cyclostoma, and other forms 

 (Simroth, loc. cit.). It is not specifically described in V i t r e 1 1 a, 

 and it is not clear from Seibold's description if it actually occurs. 



With regard to the epithelium covering this gland on the 

 renal side I do not find the condition described by Bregenzer. 

 The lining is usually a flat epithelium vnth flattened or roundish 

 nuclei (fig. 7). I have never found the peculiar epithehum 

 figured by Bregenzer (loc. cit., PL xvi, fig. 15). 



