ANATOMY OF THE OLAM. 



67 



siplion 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 fiq, ' 

 straight backwards along the dorsal side under the °^ 

 hinge and directly through the ventricle of the A, 



gills. 



heart, ending posteriorly opposite the excurrent di- 

 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, haug- 



FiG. 75. — Circulatory system of Anodonta, a fresh--n^ater mussel. 1, ventricle; 

 2, arterial system; 14 and 15, veins wliich 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 Bojanus," 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. 



ing down 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 



