THE EORING POWER OP PHOLADID.E. 29 



closes the orifices of the tubes^ and retracts them suddenly. 

 The water which they contained is thus ejected forcibly 

 from the opening in the mantle ; and the jet is prolonged 

 by the gradual closure of the valves, to expel the water con- 

 tained within the shells. The chamber occupied by the 

 animal is thus completely cleansed; but as many of the 

 particles washed out of it will be deposited before they reach 

 the mouth of the hole, the passage along which the PJwlas 

 projects its siphon is constantly found to be lined with a' 

 soft mud. 



In further apparent support of the idea of the holes being 

 made by the mechanical action of tlie valves, it has been 

 noticed that some of them are marked by rough grooves, 

 such as might be produced in that manner. On the other 

 hand however, it may be observed that the ' rasping' points 

 on the surface of the valves are never worn down, which 

 must be the case if they were constantly employed on hard 

 substances, as suggested; that in some of the species the 

 shells are externally quite smooth, and that the holes, in- 

 stead of being quite cylindrical, are fitted to the shells in 

 such a manner as, at least wlien adult, to deprive them of the 

 power of lateral action. 



The second theory, namely, that which endeavours to ac- 



