148 DIVERSIONS OF A NATURALIST 



a common parentage or ancestral relationship in the 

 forms which possess it. 



The cockle (Fig. 1 9, d), to which we now turn, has not 

 got a ribbon-rasp, nor anything of the kind. It has a 

 mouth with four flapper-like lips, but no projecting head, 

 no eyes, no biting mechanism, nor have any of the 

 bivalves, excepting a few which like the scallop have a 

 series of eyes on the edge of the soft mantle or flap 

 which lines the shell. This constitutes a greater difference 

 between bivalves and the univalves than does the shape 

 of the shell. They are a very quiescent, peaceful lot, 

 feeding on microscopic floating plants (diatoms and 

 such), which are drawn to the mouth by currents of 

 water set going by millions of vibrating hairs arranged 

 on four soft plates hanging under the protecting arch of 

 the shell, and called in the oyster — in which bivalve most 

 people know them — the " beard." 



The limpet adheres to rocks by a great disk-like mass 

 of muscle, which is called " the foot." It is really the 

 whole ventral surface, and it can loosen its hold, and, by 

 curious ripples of contraction, cause the animal to creep 

 or glide over the rock. At low tide the limpet is exposed 

 to the air, and remains motionless, but when the tide is 

 up it makes a small excursion in search of food, never 

 going more than a foot or two from the spot which it 

 has chosen, and returning to it, so that in the course of 

 time it actually wears away a sort of cup or depression 

 at this spot — if the rock is not of exceptional hardness. 

 The word " foot " is applied to the ventral disk-like 

 surface of the limpet, because in many univalves this 

 region becomes drawn out, and is connected by a com- 

 paratively narrow and nipped-in stalk or pillar with the 

 rest of the animal. This occurs in the univalves which 



