XI 



PHYLUM MOLLUSC A 



265 



foot ; by its means the animal is able slowly to plough its 

 way through the sand or mud. When the Mussel is 

 irritated, the foot and siphons are withdrawn and the valves 

 tightly closed. In a dead animal, on the other hand, the 

 shell always gapes, and it can then be seen that each valve 

 is lined by the corresponding lobe of the mantle, that the 

 exhalant siphon is formed by the union of the lobes above and 

 below it, and is thus an actual tube ; but that the boundary 



dp.ci 



FIG. 146. Anodonta cygnea. The entire animal. A, from the left side ; B, from 

 the posterior end; d.p,a. dorsal pallial aperture; ex.spJi. exhalant siphon; 

 ft. foot ; in. sph. inhalant siphon ; lg. ligament ; in. mantle ; ;//. umbo. (After 

 Howes.) 



of the inhalant siphon facing the gape of the shell is simply 

 formed by the approximation of the mantle lobes, so that 

 this tube is a temporary one. 



The hinge of the shell is dorsal, the gape ventral, 

 the end bearing the siphons posterior, the end from which 

 the foot is protruded anterior : hence the valves and mantle- 

 lobes are respectively right and left. 



In a dead and gaping Mussel the general disposition of 

 the parts of the animal is readily seen. The main part of 



