KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 59. N:0 3. 35 



Aorta posterior gives off a ventral branch between the upper ends of the ne- 

 phridia dowmvards along and between their pericardial tubes. It gives branches 

 to the interiör of the adductor and to the visceral ganglion. With its principal 

 trunk the posterior aorta follows closely beneath the rectum on the back side of 

 the adductor, until ultimately it divides into two vessels, which penetrate into 

 the siphonal muscles. 



Of the veins the most noteworthy are two large vessels running at the sides 

 of the oesophagus. They carry the blood from the anterior parts of the mantle, 

 and near the cerebral ganglia they receive branches from the foot and the liver. 

 These two anterior venous trunks run closely beneath the affixation suture of the 

 anterior (inner) gill and pass, posteriorly, into the smaller vessels enclosed in the 

 upper margin of the reflected lamina. This marginal vein gives off branches into 

 the gill along the principal filaments in a manner described under the heading of 

 the respiratory organs; on the other hand the marginal vein communicates with the 

 branchial vein in the axis of the gill. 



From the posterior parts of the body the blood is carried by a short vein 

 under the pericardium to the sinus venosus or renalis, which spreads between and 

 round the nephridia. From here the branchial vein leads through the branchial axis, 

 giving off branches on the one hand to the anterior gill by establishing a few Com- 

 munications with its marginal vessel, on the other hand to the posterior gills, where 

 these branches get the nature of filamentar vessels (see further under the heading 

 of the gills); their distal ends debouch into a posterior marginal vessel (at the edge 

 of the reflected sheet), which emanates directly from the sinus venosus. The branch- 

 ial vein is prolonged upwards outside the auricle, but does not eommunicate 

 with its interiör. 



No direct communication exists between the arterial and the venous blood 

 systems, as no veins joining the sinus venosus and the auricle are present ; the only 

 mode of transition of the blood from the veins back to the heart is the indirect one 

 by means of the branchial capillaries. From these the blood is led into the arteria 

 branchialis and then directly into the auricle. There exists, consequently, in the 

 present form a complete arterial heart contrary to the case in Anodonta, where a 

 direct communication from sinus venosus to the heart has been described (ef. 

 Hescheler 1900; Schwanecke 1913). 



The foot consists of an exteriör layer of circular träns versal and longitudinal 

 muscles crossing each other; the interiör is occupied by a lacunar tissue of thin 

 transversal muscles penetrated by the pedal nerve cords. Nearer the end of the 

 foot there appear unicellular glands in the epithelium. A byssus gland is absent. 



The muscular layer spreads backwards över the visceral pouch and forms the 

 two posterior retractors which penetrate the nephridia and are inserted on the front 

 side of the adductor. Towards the front the muscles form some fascicles which re- 

 present a pair of anterior retractors, reaching to the vicinity of the mouth only 

 and not inserted in the shell. 



The nervous system (fig. 30). The cerebral ganglia are situated us usual at 



