OYSTERS 131 



in from below) are the flat gills or " gill-plates " of the 

 mollusc (g* to g* in Fig. 28); whilst we must suppose 

 the twenty middle leaves to be " pulped " and fused 

 together to represent the body of the shell-fish. 



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 proto- 

 plasm, 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 



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 T^-gth 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). 



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 micro- 

 scopic 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 



