PHOLADID^. 



327 



the present state of our knowledge, the explanation of the learned 

 naturalist is the most reasonable which can be given. 



The engraving (Fig. 130) represents P. dactylus, which has hol- 

 lowed itself a home out of a block of gneiss. This dwelling is a cell 

 just deep enough to contain the animal and its shell, as represented 

 in Fig. 131. To excavate its cell at the bottom of one of these gloomy 

 retreats seems to be all that the animal lives for. To ascend to the 

 summit or sink to the bottom of their narrow house makes up all the 

 accidents of existence to these strange creatures : the hole they dig is 

 at once their dwelling and their grave ; which is attested both by the 

 rocks of the past and the present. 



In its structure the shell differs notably from other Acephalous 

 Molluscs, which led Linnaeus to place it with the multivalve shells. 

 Between the two ordinary valves, in 

 short, this shell presents certain acces- 

 sary pieces, smaller than the true 

 valves, and placed near the hinge, as 

 represented in Pholas dactylus (Fig. 

 131), pieces which would not be there 

 without a purpose. 



The shell is equivalve, gaping on 

 each side, swelling below, very thin, 

 transparent, and white. The animal 

 is a thick, white, elongated, fleshy 

 body ; its mouth opening anteriorly, 

 throws out a long tube traversed by 

 two canals or syphons, through one 

 of which the water necessary for the 

 respiration of the animal is absorbed, Fig - 131 - Pnolas dactylus (Linnaeus), 

 and ejected through the other. Through another opening in the 

 mantle a very thick and short foot is protruded. 



There are three ways of accounting for this creature's method of 

 boring the mechanical, the chemical, and the electric ; the first being 

 the one generally held. In this case the animal uses its foot as a 

 boring tool. The second presumes on the Pholas secreting an acid 

 which corrodes the rock; the third that it possesses a galvanic battery 

 with similar powers. It is possible that all these three theories may 

 have a measure of truth. That the foot of the borer is used is clear. 



