Page:Natural History, Mollusca.djvu/263

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251
PEARL SHELLS.

Mr.Couch, in his "Fauna of Cornwall," observes that "this species is found in the greatest abundance at the distance of from three to six leagues south of the Deadman Point, where they stud the bottom in multitudes, with only two or three inches of the pointed end inserted into the soil. It is common for the line or hook to become entangled among these shells, and powerful effort is required to drag them from their attachment, which is only effected by breaking the byssus, or tearing away the ground to which it is attached. In the latter case a rich harvest of shells is often afforded, but the pointed end of the Pinna is usually broken off by the violence. It is perhaps owing to the different degree of solidity of the ground, that the shells living in the deeper water are so much less buried than those of which Montagu speaks, and one of the consequences may be a greater degree of motion in the shell. Montagu observes that the exposed end cannot be closed by art, but the animal is capable of effecting it; and observation has taught me, that this is its method of obtaining food. In its ordinary position this opening is about two inches wide, exposing the contained animal, which occupies but a small portion of the cavity, and seems to offer itself a prey to the first creature that may choose to devour it. Some fish is thus tempted to enter, but the first touch within is a signal for its destruction. The shell closes not only at the side but at the top, the latter action being effected by a separation of the pointed ends, and the captive is either crushed to death, or soon perishes from confinement."