Page:Natural History, Mollusca.djvu/320

 be the means by which the larvæ effect their first lodgment; but, considering the arrangement of the parts of the body in the adult animal, it seems to us that Mr. Garner's view of their being the primary cause of the perforation, whilst the rasping of the valves is secondary, should be reversed. Such currents must be most effective in clearing away loosened and loosening particles. If there be any chemical action aiding, it must be due to the carbonic acid set free during the respiratory process. Evidence of a secreted solvent there is none."

To the same authority I am indebted for another interesting fact in the history of this genus.

"A remarkable property of the animals of this genus, and one which has long attracted notice, is their phosphorescence when placed in the dark. This phenomenon is exhibited by some other acephalous mollusks, and by the compound tunicated genus Pyrosoma. The light is of a bluish-white hue, and is regarded by Mayer to proceed from a luminous mucus, like that given off by the Medusæ. This mucus is thrown off into the surrounding water, so that the currents proceeding from the animal are luminous. Dr. Coldstream states, that the light is given out most strongly by the internal surfaces of the respiratory tubes, and that it is strongest in summer. Professor John Müller has observed, that when Pholades are placed in a vacuum, the light disappears, but reappears on the admission of air; also that, when dried, they recover their luminous property on being rubbed or moistened."