ANATOMY AND PHYSIOLOGY OF ATRINA RIGIDA. 
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tract, to some extent, as well as to the reproductive organ and liver. The middle one 
lies deep within the visceral mass and follows closely the coils of the intestine. The 
most anterior one, besides giving branches to the stomach and liver, gives one to the 
byssal apparatus and posterior retractor muscles of the foot (fig. 18, a, pi. xlix). Other 
arteries shown in this figure have already been described as belonging to the right side 
of the body. The main branches of the arteries are constant in number and portion, 
but the smaller ones are not so constant. There is so much variation in these as to be 
confusing to one who is studying them. The figures were drawn after dissecting several 
specimens, so that they may fairly be considered typical. 
VENOUS SYSTEM. 
The venous system, unlike the arterial, seems to be absolutely symmetrical, so 
that a description of one side will suffice for both. The venous blood enters the kidney 
from the visceral mass through a large vein which runs diagonally over the surface 
of the posterior retractor muscle of the foot. This vein brings blood from nearly all 
parts of the body, including the foot, byssus, liver, reproductive organ, and digestive 
tract. (Fig. 19, v, pi. xlix.) There is no venous sinus below the pericardium into 
which the blood collects previous to entering the kidney. The blood enters the kidney, 
as stated above, through a large vessel which breaks up into a closed capillary system. 
After bathing the glandular cells of the kidney the blood is collected into a large vein 
which transports it to the gills to be aerated. 
The blood from the adductor muscle, and probably also the mantle gland, drains 
into a sinus located on the ventral surface of the adductor. This sinus communicates 
with the vein to the gills on each side just as it emerges from the kidney, and hence 
the blood from the adductor and mantle gland does not enter the kidney, but goes 
directly to the gills. Since this sinus communicates with both sides it is possible to 
inject the veins of both sides from one point. 
Besides the sinus just mentioned there is another at the base of the foot. Those 
organs which are extended by blood pressure therefore have sinuses in their immediate 
vicinity. 
After traversing the gills the blood is returned to the heart by two vessels, one of 
which lies in the suspensory membrane and carries the blood from the anterior half of 
the gills, the other bears the same relation to the posterior half of the gills as the first 
to their anterior portion. These two vessels flow together to form a single short wide 
tube which runs at right angles to them and communicates with the auricle. This 
tube lies just anterior to the adductor muscle and runs over the outer surface of the 
retractor muscle. (Text fig. 9, and fig. 16, pi. xlviii.) Just before entering the auricle 
it receives a vein from the mantle. The latter is formed by the union of two mantle 
veins, one from their anterior and the other from their posterior portions.® 
a A number of small vessels from the palps enter the distributing vessel of the gills. The direction of the flow of the blood 
in these vessels was not deter min ed, but it is probably toward the gills. They may therefore be considered as part of the venous 
system. 
