a4 ZOOLOGY. 
masses. There i& no pharynx armed with teeth as in the 
Cephalophora and Cephalopoda, but the cesophagus 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 siphonal openings 
above the level 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 branchie 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- 
al 1q ated immediately under the hinge ; it consists 
H wtv tai of aventricle and two auricles ; the former is 
i i easily recognized by the passage through it of 
i | the intestine (Fig. 155, D, v), usually colored 
Fie 187.—Heart Gark, and by its pulsations. The two wing- 
of the clam. like auricles are broad, somewhat trapezoidal 
ventricle; A, au- ’ ‘ 
ricless @, basect in form. Just behind the ventricle is the so- 
Morse.” 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 
pranchial veins. The kidney, or “organ of Bojanus,” is a 
large dusky glandular mass (Fig. 158, 4) lying below but next 
