systems, or in some cases through both together. In the more archaic prosobranchs, the blood was primitively carried forward from the subrenal sinus to the pallial roof for re-oxygenation, by two symmetrical veins, of which only one survives in the Monotocardia, forming the longitudinal rectal sinus of the right side (see Pelseneer (1906), p. 100). A large part of the blood, however, passes through a renal portal system into the “kidney”, and this is the case even in such archaic forms as Haliotis (Crofts, 1929), in which by far the larger volume of the venous blood goes to the right renal organ. The rectal sinus of the right side, opening directly into the basibranchial sinus, is here very small. The efferent renal vein of the right side may either join the rectal sinus, or enter into the venous anastomosis in the hypobranchial gland. In a few cases, it carries blood directly to the afferent ctenidial vein.

There are four paths by which—in Struthiolaria—the blood from the subrenal sinus ultimately reaches the auricle. As compared with Dakin's account of Buccinum (1912) it will be seen that route (1) is relatively much more important in Struthioloria as compared with route (3). The by-passing in whole or in part of the renal organ appears to be a character of specialised monotocardians; and in Struthiolaria and other ciliary feeders, where the gill is extremely large, there is a functional explanation for a large direct supply of ctenidial blood, independent of blood from the renal organ.