and on the removal of portions of their Shells. 229 



that a hard shelly or spinous surface is not necessary to reduce 

 the substances into which these animals penetrate, and is strongly 

 in favour of the opinion that the valves are not instrumental in 

 boring. 



Many naturalists, feeling convinced that the shell of these ani- 

 mals is too soft to cut or rasp down the hard substances into 

 which they excavate, and being anxious to explain the pheno- 

 menon, have had recourse to the theory of a solvent. The pre- 

 vailing opinion however is, that whilst Teredo and Pholas bur- 

 row by the rasping power of their valves, Saxicava and its allies 

 penetrate by the aid of an acid solvent secreted by the animal. 

 Mr. Osier maintains this opinion in his memoir on the subject 

 published in the ' Philosophical Transactions ' for 1826, but 

 entirely failed to detect the least trace of such an acid after 

 the most careful investigation. I have also endeavoured to 

 ascertain the presence of a solvent of this nature, but equally 

 without success- After having determined, beyond a doubt, that 

 the anterior portion of the animal is the boring instrument, as 

 will be afterwards shown ; and presuming, if an acid existed, that 

 it would be secreted by follicles in the skin of this part, I re- 

 moved 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 con- 

 tact 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. 



Mr. Osier candidly acknowledges that if Saxicava were ever 

 found to penetrate non-calcareous substances, it would be fatal to 

 this theory. Now, several observers have stated, and Mr. Garner 

 and Mr. Clark are amongst the number, that this species does 

 not confine its operations to calcareous bodies. I have not been 

 able to satisfy myself on this head ; but Clavagella, which has the 

 mantle closed and thickened in front as in Saxicava, and which 

 cannot be supposed to bore by the action of the valves, has been 

 found, according to Mr. Broderip, " in siliceous grit like that of 

 the coal-measures." Few will doubt that the instrument is the 

 same in these two genera ; and if we are to believe that all the 

 Acephala excavate by the same means and that a solvent is the 

 agent, then we must allow that its powers are of the most extra- 

 ordinary nature ; for whilst it must be able to dissolve limestone, 



