MOLLUSCA. 59 



chemical and the mechanical*. The advocates of the former 

 seem to take for granted that the borings occur cliiefly in 

 calcareous substances ; and with propriety, therefore, they make 

 their solvent an acid. It is clear, however, that this agency 

 is unable to account for the abundant perforations in media 

 totally impervious to such action. Moreover no trace of acid 

 IS found in many borers ; and though present in some, as in 

 Sabella saxicava and Pholas, it is likewise cliaracteristic of 

 other marine animals which do not bore ; and it is purely 

 hypothetical at present to bring in the aid of the carbonic 

 dioxide derived from sea-water, for the same reason. 



The mechanical theory, again, supposes that the animals 

 perforate by means of shells or gritty particles in the case of 

 moUusks, teeth in the sea-urchins, bristles in the annelids, 

 horny processes in certain ciiTipedes and gephyreans ; but we 

 are left in doubt concerning the extensive and wonderful per- 

 forations of the sponges, those of the Bryozoa, and the rest of 

 the cirripedes. If, however, we regard the "macerating" 

 theory as a modification of this, certain of the difficulties will 

 be overcome. The grains of wood, however, found in the 

 stomachs of Teredo, are interesting in this respect. 



The whole subject of the boring of marine animals, indeed, 

 is much in want of further elucidation ; and it is hard to believe 

 that the same modus operandi is employed by all. In con- 

 clusion, the theories may be arranged under the followino- 

 heads (for all the subkingdoms), after Forbes and Hanley 

 and Gwyn Jeffreys : — 



I. That in the shell-fish the perforations are made by rotations 

 of the valves, like augers (Bonanni, Adanson, Born, 

 J. E. Gray, Dr. Fleming, Osier, Forbes and Hanley, 

 Cailliaud, Robertson, &c.) ; in the sea-ui-chins, by the 

 teeth (Cailliaud). 



This theory is not supported by an examination of the 

 perforations of the sponges, Bryozoa, those of the annelids, 

 gephyreans, and cirripedes, nor by a comparison of the 



We do not here allude to the boring by jaws or tongue (e. tj. in Lwt- 

 noria and Trochus). 



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