Anatomy and Physiology of Anodonta Fluyiatilis. 189 



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portions of the body ; the main artery continues posteriorly, dividing 

 into three large branches, continuing over and between the folds of the 

 intestine. The posterior aorta (PI. 10, p. ao. ), leaves the ventricle be- 

 low the rectum, and just before reaching the posterior adductor mus- 

 cle it bifurcates, and sends branches to the posterior adductor muscle, 

 to the pericardium, and to the rectum. The larger branch passes 

 over the posterior adductor muscle entering the mantle, and con- 

 tinues in the thickened portion of the mantle below the pallial line 

 until it meets the branch from the anterior aorta. The blood is 

 returned in the body by innumerable small veins, gradually growing 

 larger toward the vena-cava. For a short distance before leaving 

 the body there is a large sinus which is formed by the union of 

 the larger veins, these connecting with the innumerable small veins ; 

 this sinus connects with a cavity situated beneath and between the two 

 portions of the renal organ, which is known as the vena-cava, which 

 connects with veins and capillaries of the gills. By the contraction of 

 the ventricle the blood is forced through the anterior and posterior 

 aortae to the extremities, and is returned by the veins to the vena-cava ; 

 it then passes through the tissues of the glandular sac of the renal 

 organ ; it there loses its urea; the renal organ performing the same 

 office as the kidneys of vertebrated animals, thence passing through 

 the gills, and in the gills through a fine net- work of capillaries, and by 

 contact with the air in the water becoming oxygenated, the gills per- 

 forming the same functions as the lungs of vertebrated animals. The 

 blood circulating through the thin portion of the mantle also becomes 

 oxygenated to a certain extent, the action of the capillaries of the gills 

 being supplemented by those of the mantle ; from the capillaries of the 

 gills the blood returns to the auricles and from them to the ventricle. 

 The body sinus connects with the vena-cava by an oval opening, which 

 is covered by a projection from the anterior side, which is held in 

 place by a muscular cord ; this arrangement of the valve permits the 

 flow of blood from the sinus to the vena-cava, but effectually prevents 

 any return. The auricles are connected with the ventricle by an oval 

 opening; the " lips" of this orifice are very thin and extend into the 

 ventricles (PL 8, fig. 4), they are in shape like a sack, with both ends 

 open. At the orifice they are kept apart by their attachment to the 

 margins of the orifice, but as they recede the two margins fall together. 

 "When the auricle contracts, the blood easily passes through this valve, 

 but when the ventricle contracts, the margins fall together, preventing 

 the return of the blood to the auricles and causing it to be forced 

 through the aortae. 



The pericardium has direct communication, both with the blood sys- 

 tem and also with the branchial chamber, and thus contains a mix- 

 ture of blood and water. 



