Page:Natural History, Mollusca.djvu/233

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221
BIVALVE MOLLUSKS.

dead at least fifteen hours, yet when I placed the torn fragments of the branchiæ, one after another, beneath the microscope, the energy of the ciliary action, as the wave flowed with uniform regularity up one side and down the other of every filament, filled me with astonishment. Even the next morning, twenty-six hours after death, when the tissues of the filaments were partially dissolved, the ciliary motion was still going on, on portions that preserved their integrity.

In a sort of hood formed by the union of the gill-leaves at their basal part, is placed the entrance of the stomach; a simple orifice without jaws, teeth, or tongue, but bordered by four thin, membranous lips.

The force with which the valves resist our attempts to open them, during the life of the animal, depends on the presence of a large and powerful muscle, which is very apparent when they are opened, as it occupies a considerable portion of the interior, stretching directly from one valve to the other, and inserted by a broad base into each. By the contraction of the fibres which compose this muscle, the valves are strongly pulled together, and it is by cutting across this with an inserted knife that an oyster is commonly opened. When death ensues, the valves open spontaneously from another cause; muscular contraction then ceases, and the relaxed fibres can no longer resist the expansive force of a dense and highly elastic substance placed at the back of the shell close to the hinge, and known as the ligament.

Around this great muscle are grouped the stomach, liver, intestine and other vital organs; while in the neighbourhood of the mouth there is