Page:Natural History, Mollusca.djvu/20

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MOLLUSCA.

even before my shadow could pass over them, they were aware of my presence, and endeavoured to withdraw into the shell. I then cut off the eyes, with the thick cartilaginous tentacula in which they were lodged, but the animals still continued to be sensible of my near approach, while hanging in this mutilated and painful condition." Dr. Johnston records a manifestation of feeling, somewhat analogous to this, in one of the common shellfish of our sandy shores. "On a summer evening," he remarks, "I have observed the common Spout-fish (Solen siliqua), extended along the surface of the fine sand in which they burrow, enjoying, apparently, the calmness and mildness of the season, take alarm and instantaneously descend when I was yet distant several yards: and I can explain this and similar facts only on the supposition of the existence of a sense of touch feelingly alive to impressions impalpable to our grosser sense."

The respiration is aquatic in most of the Mollusca. The breathing organs, in most cases, resemble in essential points the gills of fishes, consisting of a great number of leaves, often minutely subdivided. They are chiefly formed of blood-vessels, covered with rows of vibrating cilia, by the constant motion of which, currents of water are perpetually hurled along the entire surface of the breathing organ, communicating oxygen, the vital principle, to the blood as they go, through the thin walls of the vessels. In many species, as the Bivalves, the gills form two large comb-like plates; in others they are arranged in the form of a feather; a beautiful tribe, known as naked-gilled,