94 THE FISHING INDUSTRY 



the surface. The oyster lives on stones and shells below 

 low-water mark. 



All molluscs are attached tightly to the shell at one or 

 two points, and cannot be removed from the shell alive. 

 In the case of the bi-valves the animal is attached to 

 the two shells by a muscle which draws the two valves 

 of the bi-valve together. When this muscle is relaxed, 

 for example in normal circumstances, when feeding at 

 the bottom of the sea the shell remains open. Some 

 shellfish notably the scallop actually swim by opening 

 and shutting the two valves of their shell. 



The most important uni-valves are the periwinkle, 

 the limpet and the whelk. Uni-valves possess a well- 

 marked head and neck, a pair of eyes and a mouth. 

 They are remarkable for the possession of a tongue, 

 formed like a ribbon rasp, furnished on its upper surface 

 with a large number of small teeth. The number and 

 arrangement of these teeth differ in different species. 

 With this ribbon rasp the uni-valve, for example a 

 dog-whelk, can rasp a hole through the shell of an 

 oyster and feed upon the contents. 



Bi-valves do not possess a ribbon rasp, neither have 

 they a projecting head, nor in most cases any eye. 

 They possess a mouth, furnished with four flapper- 

 like lips or gill plates. They feed on microscopic, 

 floating plants that are drawn within their mouth by 

 currents set up in the water by the rhythmic vibra- 

 tions from three to four hundred strokes per minute 

 of millions of hairs that hang down from soft plates 

 supported under the protecting arch of the shell and 

 called the " beard." These currents of water not only 

 bring food to the mouth of the bi-valve, but also irrigate 

 the gill plates and so enable the animal to breathe. 

 The oyster lies on the sea bottom with its muscle relaxed 

 and its shell gaping. 



