Burrowing and Stationary/ MoUusca, 359 



^owed the slightest mark of friction, and the cuticle of the 

 sides in contact was as perfect as usual. 



What, then, is the power which the Saxicava and its con- 

 geners employ? The question has not been perfectly re- 

 solved ; but it is probably an acid excreted by the animals, 

 capable of softening or dissolving lime. An objection to this 

 may be taken from some facts already mentioned, viz. that 

 these shellfish are sometimes found in argillaceous as well as 

 in calcareous rocks. The facts, however, admit of explan- 

 ation ; for the young animals may be supposed to fix them- 

 selves in holes or crevices convenient for their purpose, and 

 which afford them immediate shelter. Hence they are occa- 

 sionally found lodged among the entangled roots of sea- weed; 

 a-nd they will sometimes find a shelter in rocks upon which 

 they are unable to act chemically. And that this explanaticMi 

 is correct may be proved by the examination of the cells, 

 which are not smoothed and fashioned to the shape, as they 

 are when excavated in limestone : and, indeed, when burrow- 

 ing in the latter, if the animal meets with a piece of clay or 

 feldspar, its progress is immediately stopped, or the shape of 

 the shell is deformed by the pressure of this insoluble sub- . 

 stance. The cells in the pillars of the temple of Jupiter 

 Serapis afford examples of this fact, and Mr. Osier has 

 adduced several others which fell under his own notice. 



But this solvent of limestone must, you may still object, 

 act destructively on the shell itself, which is of the same com- 

 position, and certainly not more insoluble than the rock ? To 

 answer this, you must allow me to suppose, nor is the suppo- 

 sition an unreasonable one, that the animal has the power of 

 applying its solvent to a limited space external to the shell, 

 where it is quickly neutralised and rendered harmless. The 

 instrument of the application I believe to be the foot, an 

 organ which admits of being extended to a length fully equal 

 to that of the shell, and appears to be perforated by a tube, 

 which passes forward from the part where it joins the body 

 and terminates abruptly on the under surface near its extre- 

 mity. By this instrument the liquid can be applied remote 

 from the shell, which is thus removed from its destructive 

 influence. Where the Saxicavae are numerous, their holes 

 communicate very freely ; and it is common to meet with one 

 which has attached its byssus to another. In this case, it is 

 always found that the shell of the second has been acted on 

 in a direction, and to an extent, which corresponds with the 

 range of the foot of the assailant. The neighbouring shells 

 are very often thus corroded. " On examining a considerable 

 number," says Mr. Osier, " taken indiscriminately from the 



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