SANQUIFEROUS SYSTEM. 461 



of the systemic arteries, having their great trunks disposed 

 along the dorsal part of the body beneath the aortas, and 

 around the margin of the mantle interior to the palleal arte- 

 ries, and in a similar manner the branchial arteries closely 

 accompany the branchial veins in their distribution over the 

 laminae of the gills. Several enlargements are observed on 

 the vascular system of conchifera, as in higher mollusca, 

 and in some of the articulated tribes. The principal venous 

 trunks from the viscera appear to distribute their blood 

 through the reticulate tissue of a reniform organ, near the 

 heart, before it is collected into the two great lateral branchial 

 sinuses, like those of Crustacea or the branchial auricles of 

 cephalopods, from which the branchial arteries arise. This 

 minute distribution of the visceral venous blood, like a renal 

 portal circulation, before being transmitted to the gills, may 

 assist in its aeration or afford the materials of some secretion, 

 and the principal trunks of the palleal veins return directly 

 to the auricles, along with the aerated blood transmitted 

 by the veins of the branchiae. So that although the whole of 

 the systemic blood is not transmitted through the branchial 

 organs of conchifera the advantage of a complete double cir- 

 culation is nearly secured to these animals by the free expo- 

 sure to the surrounding element, of that portion which is 

 distributed over the extensive folds of the mantle. 



From the undivided condition of the auricle and ventricle 

 in most of the gasteropods, and the uniform systemic cha- 

 racter of these cavities, there is greater simplicity in the ge- 

 neral form of the sanguiferous system in this class than in 

 the inhabitants of bivalve shells. As they present however, 

 remarkable differences in the form, the position, and the 

 nature of their respiratory organs, and differ not less in the 

 general form of their body, these circumstances materially 

 influence the less important peripheral parts of the vascular 

 system. The respiratory organs being generally unsymme- 

 trical, and confined to one side of the body, there is no longer 

 necessity for division of the cavities of the heart, and this 

 organ is therefore commonly single and lateral in its position. 

 It consists, as seen in the annexed figure (Fig. 135.) of the 

 harpa minor, of a distinct muscular auricle, (1356.) which 

 receives the arterialised blood from the respiratory organs, 

 (135. /.) and of a systemic ventricle, (135. c.) which trans- 



