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

 Bojanus, 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 c ),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 

 Kclcr and Bojanus, 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 

 Bojanus^. 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. 



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



