ANATOMY OF THE CLAM. 
67 
siphon below the lenticular body-mass to the mouth, which is situ- 
ated at the anterior end of the shell, opposite the siphon. The open- 
ing is simple, unarmed, without lips, and often diffi- 
cult to detect. On each side of the mouth is a pair 
of flat, narrow-pointed appendages called palpi. The 
digestive canal passes through a dark rounded mass, 
mostly consisting of the liver, covered externally by 
the ovarian masses. The mouth has no teeth, and the 
oesophagus leads to a tubular stomach and intestine, 
the latter loosely coiled several times and then passing fig. 
\ 
74.— Heart 
straijrht backwards alona; the dorsal side under the ^ clam. 
° ' ® . 1 /. , ^» ventricle; 
hinge and directly through the ventricle of the A, auricles; 
heart, ending posteriorly opposite the excurrent di- giiis^^^^ 
vision of the siphon. Through the body 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, hang- 
FiG. 75.— Circulatory system of Anodonta, a fresh-water mussel. 1, ventricle; 
2, arterial system: 14 and 15, veins which follow the border of the mantle. 
The veins lead the blood in part directly towards the organ 4, which is the 
kidney or "organ of Bo j anus," and in part to the venus sinus of the upper 
surface of this organ; 5, veins which carry back the blood from the gills, the 
rest going to the sinus, 6, where arise the branchial arteries; 7, 8, the bran- 
chial veins, and 9, the gill; 16, the foot. 
mg dow^n and covering each side of the body (Fig. 72, D, g). The 
heart (Fig. 74) is contained in a delicate sac, called the pericardium, 
and is situated immediately under the hinge; it consists of a ven- 
tricle and two auricles; the former is easily recognized by the 
passage through it of the intestine (Fig. 72, D, v), usually col- 
ored dark, and by its pulsations. The two wing-like auricles are 
broad, somewhat trapezoidal in form. Just behind the ventricle is 
