158 THE BORING MOLLUSCA. 



motion ascribed to it ; that the shell cannot, by any amount 

 of this action, form the cell of the figure it really has ; that 

 the armature and natural coating of the shell, as also the 

 coating of soil often found on it, are adverse to the mechani- 

 cal theory; and that the operations of the very young, and 

 the shape and proportion of the cell in the adult, are equally 

 contradictory of it. In a similar manner Mr. Hancock re- 

 views the chemical theory, to which the great objection has 

 ever been the non-existence of any acid in the organs of the 

 animals that could be employed in secreting it or applying 

 it. Mr. Hancock has not only been unable to discover any 

 acid, but his ingenious and patient experiments demonstrate 

 that there is none to discover. After having determined, 

 he says, beyond a doubt, that the anterior portion of the 

 animal is the boring instrument ; " and presuming, if an 

 acid existed, that it would be secreted by follicles in the 

 skin of this part, I removed it from the living animal, and 

 placing the part so removed on litmus paper, pressed it 

 gently between two pieces of glass, so as to force the fluids 

 out of it. This experiment I have frequently repeated, 

 but never succeeded in detecting an acid. Another method 

 was also adopted for this purpose. Several specimens of 

 various growths were taken from burrows, and placed in a 

 vessel of fresh sea-water with the anterior portion of the 

 animal in contact with litmus paper. Here they remained 

 for upwards of a week : three or four attached themselves 

 by their byssus to the test-paper, and continued so with the 

 excavating portion of the animal resting upon it for several 

 days ; but the result was again negative — not the slightest 

 stain was apparent." 



The original theory* of Mr. A. Hancock well explains 

 all the phenomena of the question, and the structure on 

 which it mainly rests is found in all boring mollusca. Of 

 the acephalous kinds which bore in wood and in various 



* Ann. and Mag. Nat. Hist. xv. 114. Feb. 1845. Messrs. Forbes and 

 Hanley have made some objections to Mr. Hancock's discovery, which, I 

 think, further research will prove to be ill founded. The reader will find in 

 their work, a very interesting sketch of the history of opinions in regard to 

 the boring faculty of these mollusca, and some facts derived from sources to 

 which I have not access. They are of opinion that tbe mollusks excavate 

 their cells principally by a rotatory motion of the shell, produced by ap- 

 propriate muscles, and aided by currents of water set in motion by cilia. 

 The discovery of a peculiar structure of tbe shell of boring mollusks, by 

 M. Necker, viz., tbat " these shells are composed of arragonite," removes 

 a difficulty, since now " there is no reason for supposing that the shell of 

 the Pholadidae is so weak a perforating instrument as some have fancied." 

 —Brit. Mollusca, i. 96—107. 



