152 THE PIIOLAS. 



with the Teredo. These objections appearing insurmount- 

 able, a solvent acid was called in to explain the facts, and 

 when the acid was named, it was said to be the phosphoric ; 

 until Dr. Drummond suggested that the mollusks might de- 

 compose the sea-salt, as their wants required, and apply the 

 liberated muriatic acid to the solution of the calcareous 

 rock.* The existence of the solvent was at first taken for 

 granted, but when a stricter examination called for the 

 proof, it was found that there was none to be detected in the 

 animal by any test. And so another supposition was made, 

 that the acid was secreted only when wanted ; and, to 

 obviate the objection that this solvent liquor might rather 

 dissolve the shell than the rock and timber, we were told 

 that the animal had the power and organs to apply it only 

 where its instinct taught it that it could be applied to the 

 projjer purpose. Another objection, that a solvent which 

 could soften limestone and shale, siliceous grit, and clay, 

 and wood of every kind, and even wax (for a Pholas has 

 been found buried in wax), was one only to be equalled by 

 the universal elixir of the alchemists, was apparently left to 

 every inquirer's own disposal.-}- 



The question was in this unsatisfactory state, when Mr. 

 Osier attempted its solution in an essay of great interest, 

 printed in the " Philosophical Transactions for 1846."J He 

 found reason to think that both the above theories were 

 right, but neither of them universally applicable. The 

 anatomical structure of the Pholas (Fig. 27), led him to the 

 conclusion that it excavates its cell mechanically by em- 

 ploying the shell as a rasp ; and the part employed in 

 boring is the anterior or lower portion of the shell, which 



* Letters to a Young Naturalist, 230. 



t " It has been objected that any solvent which would act on a calcareous 

 rock would equally act on the calcareous shell of the animal ; but there is, 

 perhaps, more of point than of strength in this objection. Without laying 

 too much stress on that law of nature by which chemical and vital forces are 

 placed in a state of hostility, and which may or may not be applicable to 

 such a substance as shell, the gland for the secretion of the supposed sol- 

 vent, as well as the organ for applying it, may be so placed as that the 

 solvent shall only come in contact with the inorganic or dead substance to 

 be acted on, without touching the shell. 



" Again, it has been asked, what solvent would act equally on a calcareous 

 and on a siliceous substance ? To this it may be answered, first, that it is 

 not pretended that the nature of the supposed solvent is known ; secondly, 

 that in siliceoiis grits there is more or less calcareous matter by which the 

 mass is held together, and that the solution of the calcareous particles would 

 be followed by the disintegration of the stone." — Broderip in Trans. Zool. 

 Sue. Loud. i. 266. 



X Part iii. 342, " On Burrowing and Boring Marine Animals," by 

 Edward Osier, Esq. 



