[Illustration: FIG. 29.—Part of a row of the lashing hairs or “cilia” which cover the gills of the oyster. This represents part of a single row, only the 1/400th of an inch long from one end to the other. The whole surface of the gill and other parts is beset with these hairs, not in single rows, but closely, as the hairs of fur are set. The drawing is intended to show the way in which the hairs actively bend downwards (or “lash”), and then rise up again in regular waves, the movement or wave passing along in the same way as a wave of bending and returning to the upright passes over a ripe cornfield when a light breeze blows across it (see also Fig. 40).] The oyster’s gill-plates, commonly called “the beard,” are covered on the surface by microscopic hairs of a very remarkable kind (Fig. 29). They are soft, living protoplasm, and are continually “lashing,” bending forwards and straightening again at the rate of some three or four hundred strokes to the minute. They all work rhythmically together, and produce a strong current in the water, which bathes the surface of the oyster when the shells are open. Such microscopic vibrating hairs are very common in aquatic animals, and are called “cilia.” The current which they produce causes oxygen-holding water to flow from without over the gills, and so aerate the blood of the oyster, and also carries into the chamber protected by the shells excessively minute particles, chiefly microscopic plants, which are driven on to the small, open mouth of the oyster, placed far up on its body. These microscopic food-particles are wafted down the oyster’s throat by similar vibrating hairs into the stomach and intestine. An oyster has no other means of taking food, and almost without cessation, as the oyster lies on the sea bottom with its muscle relaxed and its shell “gaping,” the nourishing stream is kept going. If poisonous matter, bad water, or some violent disturbance make themselves apparent, the shell-muscle acts, and the oyster tightly closes his shell. Such things make themselves “apparent” to the oyster, for it has a nervous system, and though it has no eyes (the nearly allied “scallop” has a number of eyes) it has a delicate sense of smell and touch, and also what is usually considered to be an organ of hearing. [Illustration: FIG. 30.—The animal of an oyster removed from the shell: _a_, the thick edge of the left side mantle-flap or skirt; _b_, same of the right side; _l_, position of the mouth; _m_, shell-muscle or adductor-muscle, bringing the two shells tightly together when it contracts.]