In its structure the shell differs notably from other Acephalous Molluscs, which led Linnæus to place it with the multivalve shells. Between the two ordinary valves, in short, this shell presents certain accessary 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. [Illustration: Fig. 131. Pholas dactylus (Linnæus).] 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, 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. The luminosity which is so characteristic of the animal is in favour of an electric current, which is almost always accompanied by chemical decomposition, which would set free the hydrochloric acid of the sea water. The small size of the entrance to the chambers of the Pholas is accounted for by the increase of its size during its residence there. De Blainville thought that a simple movement of the shell incessantly repeated would suffice to pierce the stone, macerated by the sea water which passed through the breathing apparatus. Mr. Robertson, of Brighton, exhibited the living Pholas in the act of boring through masses of chalk, and thinks the process entirely effected by the simple mechanical action of the "hydraulic apparatus, rasp, and syringe."