II.] INTRODUCTION. XXxix 



4. That the animal secretes a chemical solvent — an 

 acid which dissolves the substance into which it bores. 



5. That the combined action of a secreted solvent and 

 rasping by the valves effects the perforations*.^^ 



The first of these views is advocated by Forbes and 

 Hanley; and the other naturalists to whom they have 

 referred as having expressed an opinion on the subject 

 are as follows, taking the several views in their order 

 of succession : — 1. Dr. Gray, Dr. Fleming, Mr. Osier. 

 2. Mr. Hancock. 3. Mr. Garner. 4. Dr. Gray, Dr. 

 Drummond, M. Deshayes, M. Cailliaud. 5. Mr. Thomp- 

 son, M. Necker. 



As I believe that all these different views, except the 

 first, have been successfully refuted by the arguments of 

 Forbes and Hanley, it only remains for me to adopt 

 their view, or to substitute another for it. 



If we only consider the shell of Pholas, with respect 

 to its efficiency as an instrument of mechanical perfora- 

 tion, there might be sufficient reason for supposing that 

 it can by this mode drill a hole in peat, submerged wood, 

 clay, or even in chalk when softened by the continual 

 contact of water. The shell is certainly harder than 

 any of these substances ; and the animal is pro\ided with 

 muscles of unusual strength for efiecting the rotatory 

 motion which would be necessary for such an operation. 

 But we must also consider the cases of other perforating 

 mollusks whose shells are not so hard. The shell of 

 Gastrochcena is more fragile than that of the oyster into 

 which it bores, and very much more so than the lime- 

 stone in which it not unfrequently lodges itself. The 

 helmet- shaped valves of Teredo could only be used to 

 rasp the sides of the tube which this moUusk forms in 

 wood; and they are not adapted for excavating the con* 



* ' British Mollusca,' vol. i. p. 104. 



