34 PEARLS [CH. 



of the four is but a half gill. Two half gills belong to 

 each side of the body. 



Each gill plate is made up of a number of 

 filaments, all hanging down side by side. They are 

 held together by delicate hair-like processes, so that 

 they appear to make up a firm sickle-shaped plate. 

 As a matter of fact, a slight touch with the finger 

 serves to separate the filaments so that a gill plate 

 looks like a comb, the teeth of which are not brittle 

 and strong, but soft, and only kept in position by 

 adhering to one another. Perhaps a better com- 

 parison would be with the vane of a feather. Here 

 also we have a number of delicate processes which 

 are held together to form a flat plate. 



The gills are covered with delicate microscopic 

 hair-like processes or cilia, arranged in a regular and 

 particular manner, and it is by the continual wafting 

 of these cilia that the water current enters the shell. 

 Probably all readers of this book have watched the 

 eflect of the wind, blowing across a field of corn. 

 Successive waves appear to cross the field, but the 

 motion of each cornstalk is to and fro. If one could 

 make the cornstalks move in this way, of their 

 own accord, the process would be reversed and a 

 current of air would be produced. This is exactly 

 what takes place on the gills : the cilia are the 

 individual cornstalks — they move continually, but 

 rhythmically one after the other, so that waves are 



