234 ZOOLOGY. 



masses. There is no pharynx armed with teeth as in the 

 Cephalophora and Cephalopoda, but the oesophagus leads to 

 a tubular stomach and intestine, the latter loosely coiled sev- 

 eral times and then passing straight backwards along the dor- 

 sal side under the hinge and directly through the ventricle of 

 the heart, ending posteriorly opposite the excurrent division 



Fig. 156.— Unio complanatus, partly buried in the sand, the slphonal openings. 

 above the lerel of the river-bottom. — After Morse. 



of the siphon. Through the visceral mass passes a curious 

 slender cartilaginous rod, whose use is unknown, unless it be 

 to support the voluminous viscera. The gills or branchiae are 

 four large, broad, leaf-like folds of the mantle, two on a side, 

 hanging down and covering each side of the visceral mass 

 (Fig. 155, D, g). The heart (Fig. 157) is contained in a deli- 

 cate sac, called the pericardium, and is situ- 

 ated immediately under the hinge ; it consists 

 of a ventricle and two auricles ; the former is 

 easily recognized by the passage through it of 

 the intestine (Fig. 155, D, v), usually colored 

 Fig, i57.-Heart dark, and by its pulsations. The two wing- 

 ?enSicieT"', »«' ^^^^ auricles are broad, somewhat trapezoidal 

 "fnV ^aT te°r in form. Just behind the ventricle is the so- 

 liorse." called "aortic bulb." The arterial system is 



quite complicated, as is the system of venous sinuses, which 

 can be best studied in carefully injected specimens. At the 

 base of the gills, however, is the pair of large collective 

 branchial veins. The kidney, or "organ of Bojanus," is a 

 large dusky glandular mass (Fig. 158, 4) lying below but next 



