HORNELL— ANATOMY OF PLACUNA 77 



two edges from a point dorsal and anterior where they receive blood from the anterior 

 common pallial artery, to a point posteriorly where they receive blood from the 

 posterior common pallial artery. In each case it follows that streams of arterial blood 

 must meet each other within these pallial arteries, since they are fed from each 

 extremity ; however, as these pallial vessels give oif numerous branches to the pallial 

 and velar edges and are of minute calibre, and as their blood-feeders are both among the 

 more distal of the branches given off from the aorta, the blood-pressure at that ventral 

 point where the two streams meet must be so greatly reduced as to cause no opposition 

 of blood currents to occur. 



The common palpar artery curves forwards and somewhat ventrally towards the 

 dorsal aspect of the palps, where it bifurcates to furnish a branch to each palp. 



It is worthy of note that a special supply of arterial blood is furnished to the 

 tissues in contact with the anterior and posterior thirds of the hinge line, by a 

 series of minute but very distinct arteries branching off successively at short 

 intervals from the terminal section of the anterior and the posterior common pallial 

 artery respectively. 



As already mentioned, the posterior branch of the aorta — -what may be called the 

 ventral artery (Ao.v.) — arises at a point very close to the ventricle. It runs in an 

 anterior direction for a short distance, then turns ventrally, dipping between the two 

 tubular auricles, to run along the anterior aspect of the adductor muscle between the 

 kidneys, its calibre decreasing as it goes. Opposite the anterior corner of the base of 

 the visceral mass it gives off a fairly stout branch (Art.add.), which enters the 

 substance of the adductor and splits at once into a number of dendritic twigs. From 

 this point, continuing its course along the surface of the muscle, there is next given off 

 a long branch on the right side to the sac of the crystalline style, which thence runs 

 back along the whole course of the sac, supplying branchlets thereto as it goes. 



At the level of the great parieto-splanchnic ganglionic mass three short branches 

 (fig. 15, Art.add'.) are given off successively from the ventral artery, which enter the 

 adductor muscle and split up dendritically into numerous ramifying branches and twigs. 

 Other small vessels are also derived from the extremity of the ventral artery ; two of 

 these pass posteriorly along the ventral surface of the muscle towards the rectum, and 

 it is from each of these that a branch is given off to the pallial sense-organ. 



Venous Sinuses. — Blood distributed to different parts of the body by the arteries 

 passes eventually into small ill-defined lacunar spaces, and these run together 

 into larger spaces, the sinuses, of greater definiteness and permanence. In Placuna 

 'placenta the course of the principal sinuses is more easily followed than in most 

 Lamellibranchs ; these vessels separate in the main into two series — the pallial and the 

 viscero-pedal. The blood collected by the former, as well as that from the rectal 

 sinus, passes directly to the auricles by way of the common efferent branchial 

 trunks, while that from all the viscerp -pedal sinuses, excepting the rectal, enters the 



