Glimpses of Ocean Life; Or, Rock-Pools and the Lessons they Teach

by John Harper

The foot for a long time was a complete puzzle to me: I was unable to satisfy my mind as to how it acquired its seeming extraordinary power. The phenomenon was fully explained when I became aware of the presence of that mysterious organ the hyaline stylet, situated in the centre of the foot. The use of this springy muscle, which is, as we have shown in the previous chapter, by naturalists erroneously considered to be the gizzard of the animal, is, I believe, solely to assist the Pholas in its boring operations.

Perhaps some of my readers would like to know how to procure a sight of the stylet; if so, their wishes may be easily gratified. Take up a disentombed Pholas in your hand, and with a sharp lancet or point of a pen-knife, briskly cut a slit in the extreme end of the foot, and, if the operation be done skilfully, the object of your search will spring out of the incision to the extent, it may be, of a quarter of an inch. If not, a very slight examination will discover the opal gelatinous cylinder, which may be drawn out by means of a pair of forceps.

When extracted and held between the finger and thumb by its smaller end, the stylet will, if struck with a certain degree of force, vibrate rapidly to and fro for some seconds, in the same manner as a piece of steel or whalebone would be affected, under like circumstances.[14]

So long as a Pholas exhibits only the ends of its siphons to the eyes of a greedy crab, it is perfectly safe from attack. It is only when the fleshy foot is unprotected that it falls a prey to some hungry crustacean.

The toughness of the siphonal orifices is, I believe, a most important point, for, as I shall endeavour to explain, the siphonal tubes constitute important accessory excavating agents, to those already enumerated.

We all know that the hole which each young Pholas makes, when first he takes possession of his rocky home, is extremely minute,—not larger than a small pin's head; now, it stands to reason, that if the shell was the only terebrating agent, the opening of the cavity in question would always remain of the same size, or, perhaps, on account of the action of the water, a slight degree larger than its original dimensions. Such, however, is not the case.

Here is a fragment of rock exhibiting several Pholas holes. The aperture of one of these, which I measure, is nearly half-an-inch in diameter, while in juxtaposition with it is situated another cavity, measuring across the entrance less than the eighth part of an inch. The reader will at once perceive, if the foot and shell were the sole augurs, that as the animal descended deeper into the rock, the siphonal tube, as it enlarged in proportion to other parts of the animal, would have to be drawn out to an extremely fine point to fit the opening of the tunnel. But as this is not the state of matters, the conclusion forces itself upon us, that that portion of the orifice situated above the shell of the animal must be enlarged by the constant extension and retraction of the siphons, aided by currents of water acting on the interior surface of the cavity.

This same theory will also serve to explain how it is that all Pholades situated at the same depth in the rock, are not all of a uniform size. I have frequently seen a piece of rock exhibit the peculiarity of two burrows of vastly different proportions as regards breadth, being precisely the same depth from the surface of the stone. This appears to me equally wonderful and puzzling at first sight, as the 'boring' question.

What age is attained by any species of the rock-borers before they arrive at full growth, there are no means of knowing. This point, like several others in the history of these animals, still remains a mystery, nor is it likely soon to be cleared up. The largest specimen of P. crispata that I have seen is at present in my possession. Each valve measures three and a half inches in length, by two inches in breadth. Some foreign specimens of this species, and especially of P. dactylus, are, however, frequently found of much larger dimensions.

On no occasion have I ever examined any Pholas excavation that had lost its conical shape, a fact that 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.[15] 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 necessarily present a parallel appearance—a phenomenon which is never witnessed.