Although this sweeping statement is incorrect, I will freely admit that the foot constitutes an agent second only in importance to the shell of the animal. A casual examination of any Pholas perforation will show that the foot could not have been the only instrument by which the cavity was formed, from the peculiar rings that line the lower portion of its interior. These rough appearances, I feel convinced, could be formed by no other means than the rotatory motion of the shelly valves. The valves, however, could not rotate and press against the surface of the rock, were it not for the aid which the foot affords to the animal, by its being placed firmly at the base of the hole, and thus made to act as a powerful fulcrum. This supposition fully accounts for the lowest extremity of the rocky chamber being always smooth, and hollowed out into a cup-like form by the action of the fleshy foot above alluded to. 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] [14] In the _Athenæum_ (Nos. 1632 and 1636), were kindly published two letters from the author on the above subject, under the respective dates January 26th, and February 28th, 1859. 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.