THE STRUCTURE OF THE OYSTER. 75 



Warms the new heart, and dyes the gushing blood ; 

 With life's first spark inspires the organic frame, 

 And, as it wastes, renews the subtle flame." 



Accordingly, the oyster has organs of breathing which 

 may be more readily understood by the examination, often 

 practicable, of another creature. 



Such for example, is the Pholas, of which we have 

 several British species, occasionally contenting themselves 

 with houses of clay, but commonly boring an abode in 

 sandstone or limestone rock, and, consequently, the pre- 

 cursors of the long series of cave-dwellers of which we 

 have read in various parts of the earth. Their colonies 

 may frequently be observed at low water mark in the 

 masses of stone left uncovered by the retirement of the 

 tide. How these recesses are made has long been a 

 question which men of science have laboured to answer, (d) 



The Pholadce are a family which can bore out for 

 themselves a residence in wood, or even stone. They 

 appear to carry with them a graving tool, and the shell is 

 fitted in a hole, as in a needle case. How can these 

 animals bore their way into the very hardest rocks ? 

 Aldrovand believes that they were born in the bosom of 

 the rock while it was in a soft state, and Reaumur shares 

 this extraordinary opinion ; but this cannot explain their 

 presence in logs of wood. Others have supposed that the 

 current of water produced in the process of their respira- 

 tion wore out the cavity by its continual erosion ; but this 

 theory again is not supported by fact, lor the pholas finishes 

 its excavation in a few months, which could not possibly 

 be the case if water were the only agent. Another suppo- 

 sition is, that the foot and the edge of the mantle are 

 filled with siliceous particles, like sand-paper, which, by 



(d) " Adventures of an Oyster," pp. 12-20. 



