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 observation, 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, however 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 student. I therefore managed to saw away the rock in such a manner, as to leave a narrow slit along the entire length of the tunnel, so as to expose the slightest movement of the animal within. Having natural support for its siphons, I expected that these organs would be constantly retracted and extended; but such was not the case; at least for so long a period as I had anticipated.
After repeated experiments, I have now discovered that whether the siphons be protected as above described or not, they will always be vigorously exercised if the animal be placed in shallow water, so that its tubes when fully extended will reach the surface of the fluid.
The conclusion, from what has been stated, is, that the Pholas can no longer be considered a weak and helpless animal. Possessed of a rasp-like shell, a horny ligament, retractile tubes, a strong muscular foot, and a powerful spring or stylet, it is not by any means difficult to conceive that these agents when they are all brought into play, are fully equal to the task of excavating the rocky chamber in which the animal lives.