PLANT-EATING MOLLUSCS 



197 



of the mouth. It must, of course, be clearly understood that the 

 words "jaws" and "teeth", as appHed to the biting organs of 

 snails, only refer to their use, and are not meant to suggest 

 equivalence to the similarly-named parts of back-boned animals. 



Limpets. — It would be tedious to describe in detail the feeding 

 habits of many plant -eating molluscs, and it will probably be 

 sufficient for the present purpose to illustrate the matter by a 



Fig. 436. — A Limpet [Patella vulgata) leaving its scar at ebb-tide 



few examples. The Common Limpet [Patella vulgaia) (fig. 436) 

 abounds on rocks between tide-marks all round our coasts, living 

 on a depressed spot, or "scar", worn out in the hard stone, and 

 making this the centre of feeding excursions when the tide is 

 down, returning to its home when the water begins to flow, and 

 fixing itself firmly by means of the large fleshy foot projecting 

 from the under side of the body. Numerous limpets are found 

 upon rocks closely studded with acorn-barnacles, and at first sight 

 free from plants which might serve as food. Closer inspection, 

 however, will show in such a case that rocks, barnacles, and 

 even the limpets themselves, are to a large extent closely invested 

 by a covering of minute algae. It is very interesting to observe 

 these molluscs feeding under such circumstances, a warm still 

 day being usually most favourable for the purpose. By sitting 



