24 BRITISH MARINE TESTACEOUS MOLLUSCA : 



their junction with the body, under the mantle, at the dorsal 

 range, and is of concurrent length with them ; each branchial 

 vein coasts the margins of its respective lamina. The motive 

 power of the circulation is a heart, which is a simple ventricle, 

 and two symmetrical, oblong auricles, one on each side of it; 

 it is placed subcentrally on the dorsal range, rather nearer to 

 the posterior half, and with the auricles embraces the rectum 

 or intestine, which thus apparently passes through it. But 

 this is not the case with every bivalve, though the major part 

 of them are thus constituted; the Ostrea, Area, and some 

 few others are exceptions. The heart and auricles are nearly 

 pellucid, of the thinnest texture, and apparently of feeble 

 power; the auricles have valves to prevent a reflux of the 

 blood; they are usually situate at their junction with the 

 heart, sometimes at the point where the branchial veins unite 

 with them, and they sometimes are found at both contacts. 

 The inspirations and expirations are each about seven or 

 eight in a minute. The heart gives forth at least two aortic 

 branches, which ramifying furnish arteries to the system, and 

 at their invisible termini the venous reflux has its origin, and 

 by continual anastomoses and inosculations, in their progress 

 to the arterial centre, terminate in one or more trunk veins 

 or vense cavse, which effect a coalition with the branchial 

 artery. 



Though the circulation is what is called complete, it may 

 be liable to interruptions, divergences and retardations ; that 

 is, if the opinion of some zoologists, Mr. Garner amongst them, 

 is well founded, that the whole of the blood is not returned 

 to the body in each circulation, and a part of it, from various 

 organs, passes to a set of veins which are concentrated ante- 

 riorly, on each side the dorsal range, in an elongated sinus, 

 extending to and under the pericardium ; and that these veins, 

 by small lateral internal sacs, ducts or orifices, pass the secre- 

 tions, or fluids they have distilled, to the mantle, in aid of 

 structure and coloration, for the use of the ova and other 

 organs ; and that branches from this fasciculus of veins, after 

 the depuration of their contents, transmit the blood into the 

 branchial artery; it is also said that other portions of the 



