19? THE SEAS 



during the process. Those worms which live in calcareous 

 or parchment-like tubes bear round their head end a crown 

 of foliaceous tentacles, often of great delicacy and beauty, 

 which normally wave about freely in the water, but which are 

 withdrawn in a flash on the approach of enemies (Plate 77). 

 These tentacles are covered with cilia and also wuth a sticky 

 substance in which fine particles are entangled, being later 

 carried to the mouth by currents produced by the beating of 

 the cilia. But it is the bivalve molluscs, such as the oyster 

 and mussel, which have carried this method of feeding to 

 the greatest degree of perfection. As showTi in Plate 75, 

 on either side of the body there are two pairs of dehcate 

 membranes known to scientists, owing to the mistakes of 

 earlier naturalists, as gills, and to the oyster merchant as the 

 " beard," but which really form an apparatus for collecting 

 food. Each of these membranes is double and consists of a 

 great number of fine filaments laid side by side and covered 

 with rows of cilia. The cilia on the sides of the filaments 

 beat inwards so that they cause a strong current of water to 

 pass into the interior of the membrane and so upwards and 

 out of the body. The cilia beat continuously so that as long 

 as the shell is open there is a continuous stream of water 

 entering at one point, passing through the membrane of the 

 gills, and then out at another. Only the water passes 

 through the gills ; all particles in suspension are retained on 

 the surface, where they are entangled in the sticky mucus 

 there produced and carried by the cilia on the outer surface 

 of the filaments to the edge or base of the gills, whence they 

 are conducted by further cilia to the mouth, which is 

 protected by two pairs of triangular flaps known as 

 " palps." These are ridged on their inner surfaces and 

 covered with many series of cilia beating in every possible 

 direction, the whole forming an extremely complicated, but 

 very ef&cient, mechanism for sorting out the particles. 



