THE MOLLUSCA 103 



With this huge foot the Natica slowly ploughs its way along in 

 the soft sand, making a regular furrow as it goes, in search of the 

 small bivalves that lie buried beneath the surface. With its 

 file-like tongue it bores a hole right through their shells and greedily 

 sucks out the contents. 



The Top-Shells, the Periwinkles, and the Limpets are all vege- 

 table feeders, and Use their long, toothed radula for rasping away 

 at the seaweeds which constitute their food. Many species of 

 Top-Shell (Trochus) are found in the low rock pools, in crevices 

 at the base of rocks in shallow water, and amongst the sea-grasses 

 on muddy shores. The shells are pyramidal in shape, with an 

 almost flat base, those frequenting deeper water always being 

 much more depressed than those found between tide marks. The 

 genus Trochus has a very wide distribution ; the shells are often 

 beautifully coloured and sculptured, the finest specimens being 

 found in the tropic seas. 



The Limpets, which are such familiar objects on the rocks 

 between tide marks, have no operculum; the great round foot 

 occupies the whole of the opening of the tent-like shell. With 

 this the animal adheres to its support so firmly that it is said 1 

 that a limpet with a base of an inch and a quarter by one 

 inch requires a pull of seventy pounds to dislodge it. Limpets 

 are nocturnal in their habits, like the majority of molluscs, and 

 leave their positions only at night-time to feed ; but they 

 always return again to exactly the same spot, so that in time 

 the surface of even the hardest rock becomes worn away by the 

 mollusc, and a shallow pit is formed into which the edge of the 

 shell exactly fits. The shell of the Rock Limpet is remarkably 

 hard and solid, as it has to bear the full force of the breaking 

 waves ; while other species of limpet that habitually float in the 

 water attached to the stems of seaweeds are considerably thinner 

 and much smoother in texture. 



There are many limpet-like shells called " False Limpets," for 

 although the shells are practically identical with the shells of the 

 True Limpets (Patella), the animals themselves differ in the arrange- 

 ment of the gills. True Limpets have a circle of well-developed 

 gills round the sides of the foot, on the edge of the mantle. False 

 Limpets have only a small gill placed on the left side of the neck. 



i J. Sinel 



