246 COLLEGE ZOOLOGY 



open, the valves being forced apart by the elasticity of the 

 ligamentous dorsal hinge, which is compressed when the shell is 

 closed. 



The two folds of the dorsal wall of the mussel which line 

 the valves are called the mantle or pallium (Fig. 175, m). 

 The mantle flaps are attached to the inner surface of the 

 shell along a line shown at 5 in Figure 173. The space be- 

 tween the mantle-flaps containing the two pairs of gill plates 

 (Fig. 174, 12, 13), the foot (9), and the visceral mass, is called 

 the mantle cavity. 



Digestion. — The food of the mussel consists of organic 

 material carried into the mantle cavity with the water which 

 flows through the ventral siphon (Fig. 173, 8; Fig. 174, 14). 

 The mouth (Fig. 174, 1; Fig. 175, mth) lies between two pairs 

 of triangular flaps, called labial palps (Fig. 174, 8). The cilia 

 on these palps drive the food particles into the mouth. A short 

 oesophagus (Fig. 175, gul) leads from the mouth to the stomach. 

 On either side of the stomach is a lobe of a glandular mass called 

 the digestive gland or liver (d.gl) ; a digestive fluid is secreted by 

 the liver and is carried into the stomach by ducts, one for each 

 lobe. 



The food is mostly digested and partly absorbed in the stomach; 

 it then passes into the intestine (Fig. 175, int), by whose walls it is 

 chiefly absorbed. The intestine coils about in the basal portion 

 of the foot, then passes through the pericardium (pc), runs over 

 the posterior adductor muscle (p. ad), and ends in an anal 

 papilla (a). The faeces pass out of the anus and are carried 

 away by the current of water flowing through the dorsal siphon 

 (Fig. 173, 7). 



Circulation. — The circulatory system comprises a heart, 

 blood-vessels, and spaces called sinuses. The heart (Fig. 175, 

 r.au., v) lies in the pericardium (pc). It consists of a ventricle 

 (v) which surrounds part of the intestine (ret), and a pair of 

 auricles (r.au). The ventricle by its contractions drives the 

 blood forward through the anterior aorta (a.ao) and backward 



