and on the removal of portions of their Shells. 239 



mal are furnished with the means of removing the various sub- 

 stances into which the burrows are made. The means employed 

 cannot be a solvent^ unless wc are prepared to suppose it capable 

 of eroding argillaceous, calcareous, and siliceous bodies and also 

 wood ; and we have already seen that in the case of Teredo the 

 wood is reduced to sawdust, and in no way affected by chemical 

 action ; likewise that the excavations of Patella occasionally 

 exhibit a scratched surface. This latter fact w^ould appear also 

 to exclude the instrumentality of ciliary currents, as they can 

 scarcely be deemed sufficient to produce such a surface. And 

 since we have seen that Saxicava is unprovided with cilia on the 

 boring instrument, few naturalists will be disposed to support the 

 theory promulgated by Mr. Garner : all such, however, are com- 

 pelled to allow that the mighty currents of the rolling tide are 

 far less potent than the minute breathing currents of the mol- 

 lusk ; and that they, microscopic as they are, can in the course 

 of a few months dig deep into hard shell and limestone, — nay, 

 even into the hardest marble which has withstood the shock of 

 the elements for ages. Some naturalists may still be inclined to 

 urge that one species may work mechanically and another che- 

 mically. But is it not more philosophical to allow that animals 

 so nearly allied as these — or at all events that all the boring 

 Acephala — are more likely to effect a similar purpose by the 

 same means, than that several should be adopted ? Surely this 

 is more consistent with the unity of the laws of nature, and that 

 beautiful simplicity which is everywhere prevalent in her works*. 

 We shall now examine the anterior portions of the animal. 

 The surface of the foot of Teredo norvegica when preserved in 

 spirit is tough and coriaceous, and is entirely covered with little, 

 irregular pimples. If a portion of it be placed in the compressor 

 of the microscope, it is perceived to be crowded with minute, bril- 



* Professor E. Forbes and Mr. Hanley, who, as before stated, advocate 

 the mechanical action of the shell, aided by the currents produced by the 

 animal, appear also to contemplate the probability of the assistance of an 

 acid solvent ; for they state in conclusion that " if there be any chemical 

 action aiding, it must be due to the carbonic acid set free during the respi- 

 ratory process. " The advocacy of such a multiplicity of means to effect the 

 same purpose cannot be avoided by those who believe that the Pholades 

 work out their excavations by the rasping of their valves. Saxicava has a 

 smooth shell, and is unprovided with cilia on the anterior portion of the 

 animal ; therefore the assistance of an acid solvent must be resorted to in 

 this case. But in that oi Patella, which penetrates shale and shaly sand- 

 stone as well as calcareous bodies, such a solvent cannot be supposed to act 

 for the same reason that would preclude its use in the Pholades ; and there- 

 fore in Patella the currents produced by the animal must be imagined to 

 operate, as the shell here is out of the qtiestion. Thus all these three means 

 are requisite to account for the ph.xnomenon, if the theory proposed in the 

 * History of British Mollusca ' be correct. 



