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 sui'face to a little below its middle, and then x>enetrates 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, hv, 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, Ko. 8) have corj)uscles 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 x^rotruding 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, iDalps, 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 {Xucida., Yoldia, Solenomya), each of 

 these is little more than a blood sinus ( Yoldia, Figs. 79, 80, 82; Xucula), 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, 



