CH. Vl] THE FRESH-WATER MUSSEL 167 



fixed and the shell relatively movable (vide infra, sub 

 Locomotion, p. 171). 



The mantle, to which reference has already been made, 

 is a fold of the dorsal surface of the animal hanging down 

 on the right and left sides, and conforming in extent and 

 outline with the valves of the shell. The thickened edges 

 of the folds are not united except opposite the posterior 

 adductor muscle, where they are fused for about half-an- 

 inch. Just below this region of fusion each edge is 

 pigmented, and withdrawn twice in quick succession from 

 its fellow. Two slits are thus left between the right and 

 left mantle lobes at the posterior end of the animal. The 

 lower slit, whose walls are provided with a fringe of short 

 tentacles, is the inhalant siphon through which currents 

 of water are drawn into the cavity beneath the mantle by 

 the action of cilia, which clothe all the free external 

 surfaces and are especially pronounced upon the gills and 

 labial palps. The water passes at first forwards and 

 dorsally through the lattice-work of the gills, it then is 

 directed posteriorly and eventually leaves the shell by the 

 upper slit, the exhalant siphon. 



When at rest and undisturbed in its natural habitat 

 the mussel buries fully the anterior three-fourths of the 

 length of the shell in the soft mud. Indeed it requires 

 some experience to detect their presence, for frequently the 

 only parts visible are the margins of the two widely opened 

 siphons enclosing an 8-shaped space. The " waist " of the 

 8 is formed by a small lobe of the mantle of one side 

 overlapping the other side so as, functionally, to separate 

 the exhalant from the inhalant siphon. In the necessarily 



