TEREBRATING POWERS OF THE PHOLAS. 261 



seems to prove that the successive stages of the boring 

 operation must have taken place solely in consequence 

 of the animal not having reached its adult form. 1 

 For had the shell attained its full development, and 

 its owner continued to labour, and rasp away the 

 rock, the sides of the cavity at its base would neces- 

 sarily present a parallel appearance a phenomenon 

 which is never witnessed. 



From this we may conclude that the depth of the 

 perforation, which is seldom many inches, depends 

 entirely upon the growth of the mollusc. 



When keeping specimens of the Pholas for obser- 

 vation, the usual plan is to chip away the rock to 

 the level of the valves, so that the whole of the 

 animal's siphonal tubes may be distinctly seen, how- 

 ever slightly these organs may be extended. This 

 plan, I found, did very well for a time, but I was 

 annoyed to witness, that in the course of a few months, 

 the siphons ceased to be either advanced or retracted, 

 they having become, as it were, rudimentary. 



To obviate such contingency, the writer adopted 

 the following scheme. 



To place in the tank a Pholas completely embedded 

 in a fragment of rock, so that nothing but the tips 

 of its siphons, when extended to the utmost, were 

 visible, would not afford much pleasure to the stu- 



1 The above remark holds good, even although (as is frequently the case) the ani- 

 mal wilfully deviates from the straight path, and bores its tunnel in a curved 

 form. 



