PHOLAS MUSSEL COCKLE. 75 



vent an acid which dissolves the substance into 

 which it bores; 5th, That the combined action of a 

 secreted solvent and rasping by the valves effects the 

 perforations.' 



"The most plausible reason" (says Mr Sowerby) "in 

 favour of the first of these theories, is that given by 

 Mr Osier, who describes the young animal first fixing 

 itself by its foot, raising itself almost perpendicularly, 

 and pressing the prickly part of the shell against the 

 stone in a half-rotary motion, and thus adapting its 

 motions to its advancing age, the size of the opening, 

 and the progress of the work. Mr Osier goes on to 

 say, ' The particles rubbed off, and which in a short 

 time completely clog the shell, are removed in a very 

 simple way.' When the projected syphon is distended 

 with water, the Pholas 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 contained 

 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 Pholas projects its syphon is found con- 

 stantly lined with soft mud. 



" In further apparent support of this idea of the 

 holes being made by the mechanical action of the 

 valves, it has been noticed, that some of them are 

 marked by rough grooves, such as might be produced 



