262 Introduction to Animal Morphology. 



On its upper surface are two dark-brown crescentic 

 membranous folds, pierced by fine reticular openings, 

 and surrounded by capillary lacunae (the red-brown 

 organ oi Kebcr) : these, with the ducts of the organ of 

 BojanuSy serve as outlets from the pericardium. The 

 heart has one ventricle with a spongy interior, tra- 

 versed by the intestine ; in Ungulina the latter only 

 traverses the wall of the heart, and does not enter its 

 cavity. In Area the intestine splits the ventricle in 

 two ; in Ostrea, Anomia, and Teredo, the heart lies 

 under, and is not pierced by, the intestine. Two au- 

 ricles (one in Anomia)* receive the blood from the 

 gills, and transmit it through a two-valved auriculo- 

 ventricular opening into the ventricle, from the front 

 of which arises the anterior aorta (see Fig. 3i'),t 

 whose branches supply the foot, mantle, anterior ad- 

 ductor, &c. In Anomia this vessel has an arterial 

 bulb at its base. The ventricle gives off behind the 

 posterior aorta (absent in Teredo), which passes be- 

 tween the foot muscles to the pericardium, organs of 

 Keber and Bojanns, intestine, &c. The large vessels 

 have coats and an epithelial lining, but end mostly in 

 wall-less lacunae ; sets of true capillaries, however, 

 having structureless walls with scattered nuclei, 

 exist, as an erectile layer in the foot, mantle, and in- 

 terbranchial septum, and as arborescent branchings on 

 the intestinal wall. The blood is returned by veins, 

 partly into the venous sinus, under the organ of 

 BojanuSyX or into that organ itself, or directly into 



* The two auricles unite in the oyster. 



t In Area each ventricle gives off an aorta, and the two unite to form 

 a common trunk. 



X This sinus receives one branch from the erectile tissue of the foot,. 



