HORNELL— ANATOMY OP PLACUNA 79 



would appear that some elimination of waste products may here take place. This sinus 

 receives some portion of venous blood from adjacent portions of the right mantle. It 

 runs directly to the heart, uniting with the right auricle just under the right auriculo- 

 ventricular aperture in such manner that it appears as if it were a backward 

 prolongation of this auricle, just as the right common efferent branchial trunk appears 

 as a forward prolongation. 



Within the main visceral mass the two principal sinuses consist of an asymmetric 

 pair, disposed vertically close behind the junction of the palps with the body. The 

 right (Sn.v.r.) is the larger and has many branches, which in turn frequently 

 sub-divide. These branches drain blood from the greater portion of the visceral mass, 

 including the whole of the median and right sections of the main mass; a branch from 

 the posterior visceral lobe also empties into the main or vertical sinal trunk. 



The left main visceral sinus (Sn.v.l.) lies parallel to the right one, and receives 

 branches from the anterior portion of the right side of the visceral mass and also a 

 strong branch from the foot. 



A very little above the level of the lower margin of the palps the left visceral 

 sinus turns inwards and joins its fellow on the right, forming a short median sinus 

 (Sn.m.). This almost immediately divides into two large, well-defined trunks, the 

 renal sinuses, passing ventrally to tlie paired limbs of the kidney. Here the blood 

 stream bathes the renal tubules with consequent elimination of waste products. From 

 the ventral extremity of each limb of the kidney and close to the parieto-splanchnic 

 ganglion, a common afferent branchial trunk (Cbr.aff.) conveys this partially-purified 

 blood into the branchial mesentery of its own side. The course of this afferent vessel 

 curves at first outwards (anteriorly and slightly ventrally), always within the 

 mesentery, till it reaches rather less than half-way to the proximal margin or base of 

 the gills. Here it divides into a short anterior and a long posterior branch. From 

 both branches a multitude of fine vessels carry blood from the afferent trunk into a 

 most extensive and intricate vascular network {Br.ms.pl., figs. 4 and 15), occupying 

 the whole distal section of the mesentery and from which eventually pass the 

 tubules supplying the branchial filaments. 



The remainder of the branchial circulation is practically the same as in Anomia 

 and is described in the pages treating of the structure of the branchiae. It will suffice 

 to say here that, after oxidisation in the mesenteries and the gill filaments, the blood 

 from the gills is returned to the hcarb by two common efferent branchial vessels, each 

 of which, as already noted, runs directly between the anterior apex of each gill and the 

 auricle of the same side — appearing, indeed, as a forward prolongation of the auricle. 



Course of the Circulation.- — We are now in a position to* outline the course of the 

 blood circulation through the body. In the first place, we see that the blood reaching 

 the ventricle through the two auricles consists in part only of fully-purified blood fron) 

 the gills and kidneys, a portion of more or less impure blood arriving in the auricles 



