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the epedermis onthefliellmufl firil be cleftroyed to admit the 

 liarder bodies coming in contaft. Befid^s, reafoning from 

 analogy, we find a funilar conftru6lion of parts in the Teredo 

 Gigantea, whofe habits require no fucli implements ; and 

 many other teliaceous Afciclia, wliofe habits are fimilar to 

 that of Teredo navalis, are capable of eroding the hard eft 

 lime-ftone, and even thofc compofed of fand mixed witli 

 calcareous earth. From thefe fadls \\ e are naturally led 

 to look for another procefs by -which thefe works are per- 

 formed. 



In tivs country, there are not lefs than twelve fpeciesof 

 teftaccQUS Afcidia which are perforators : all the Pholadcs 

 perforate wpod, and fometimcs the fofter ftones, and indu- 

 rated clay : Donax irus, Mytillus rugosus, Venus perforans, 

 Mya, Pholadia, diftorta, aiid fuborbicularis excavate ex- 

 tretnely hard flone, and yet there is nothing in the lljells, 

 of thefe laft that can poflibly be confidered capable of per- 

 forming fuchatalk, many of which are regular, intire, and 

 nearly Imooth, efpecially the Mya fubqrbiculam, which is 

 finely polifiied. 



Perceiving then the effect of thefe animals on fubftances 

 capable of grinding their fliells to powder by attrition, with- 

 oMt affecting the fliells, are we not nioft reafonably to con- 

 clude that they are capable of fecreting forae folvei}t men- 

 ftruunij with which alone they effe6l the purpofes of their 

 nature. ? And are not thofe two large glands, dei'cribed by 

 Mr. Home, on the right fide of the a^fophagus, in tlie animal 

 of Teredo navalis, intended for this purpofe .^ 



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