408 
BULLETIN OF THE UNITED STATES FISH COMMISSION 
runs forward to the visceral mass and mantle. The latter takes its origin slightly 
posteriorly to the first, very close to the extreme anterior end of the ventricle, and 
runs directly backward beneath the rectum and in the upper wall of the pericardial 
chamber. Upon reaching the adductor muscle it turns downward, running along its 
anterior surface to a little below its middle, and then penetrates its tissue and 
becomes distributed among the fibers. 
The oyster probably came, in its degeneration, through a form with an anterior 
and posterior aorta. But in this case the posterior aorta must have shifted its posi- 
tion from the posterior end of the ventricle to its anterior dorsal end. 
Arteries break up, not into capillaries, but into irregular blood spaces, in all 
tissues of the body which they penetrate. In the gill filaments, however, the blood 
channels are of more regular size, and in many cases ( Pecten , Fig. 83) are easily seen to 
be lined with a distinct endothelium. In the walls of the digestive tract and in the 
labial palpi also, the blood spaces are quite regular, and much like definite vessels. 
These are lined by an endothelium, as is shown in Fig. 75, PI. xc, bv, a section of the 
mouth fringe of Pecten irradians. 
The blood of lamellibranchs is colorless, with a few exceptions, and contains many 
corpuscles. Some of the Areas and Solen (Lankester, No. 8) have corpuscles contain- 
ing haemoglobin, so that the blood is distinctly red. 
The relative amount of the blood is very great in some locomotor forms where it is 
used in protruding the foot, and in such sedentary forms as Ostrea and Mytilus is com- 
paratively small. 
The course of the circulation is as follows : From the ventricle of the heart the 
anterior aorta conveys the blood forward along the dorsal wall of the visceral mass, 
over the stomach, and then down into the foot. From this main artery many branches 
are given off to the tissues of the liver, sexual glands, palps, digestive tract, and foot. 
Where the posterior aorta is present it is in most cases distributed mainly to the man- 
tle folds, and also supplies the siphons of the mantle and the posterior adductor. If 
the posterior aorta be absent, these posterior tissues are supplied by a branch from 
the anterior aorta. From the irregular sinuses into which the arteries empty, the 
blood is collected in larger vessels and conveyed to a vessel beneath the pericardium, 
called the sinus venosus. Thence it passes to the gills, traversing on the way the 
walls of the nephridia, where waste products are excreted. The circulation is com- 
pleted by the return of the blood from the gills to the auricles of the heart. 
The path of the blood through the gill filaments is not well known and would be 
impossible to determine in those forms in which the gills have become greatly special- 
ized, owing to their complex form. On account of the colorless condition of the blood 
corpuscles also, their movement in the filaments can not be followed. 
It seems altogether probable that the manner of the circulation in the gills is 
very dissimilar in different groups of lamellibranchs. In those forms in which the 
gills are made up of a series of leaf-like plates (JYucula, Yoldia , Solenomya), each of 
these is little more than a blood sinus ( Yoldia , Figs. 79, 80, 82; JYucula ), around whose 
outer edge a blood channel is more distinctly marked out. While a circulation may 
be more or less distinct here, it can not be perfectly so. 
In forms with a descending and ascending portion in the filament, and where the 
latter is not in concrescence at its extremity with the mantle or neighboring filaments, 
the blood must flow out to the extremity of a filament and then back again, perhaps, 
