MARINE MOLLUSCS 



239 



upper jaw, and the tongue, which is very long, is supplied with 

 numerous hooked teeth. The foot is a very large disc, as large 

 as the shell, and the gills consist either of one or two branched 

 plumes, or of a series of lamellae almost or entirely surrounding 

 the animal between the shell and the margin of the mantle. 



The reader has probably experienced the difficulty of detaching 

 a limpet from its hold on the rocks. The tenacity of the grip is 

 not due to the mere adhesive power of the foot itself, but to 

 atmospheric pressure, the effect of which is complete on account 

 of the total exclusion of air from under the disc of the foot ; and 

 when we remember that this pressure amounts to fifteen pounds 

 on every square inch of surface, we can readily understand the force 

 required to raise a large limpet from its position. 



123 



FIQ. 166. PatellidcB 

 1. Patella vulgata. 2. P.pellueida. 3. P.athletiea. 4. Acmaca tettudinalit 



The Common Limpet (Patella vulgata) is found on all our 

 rocky coasts between the tide-marks, often at such a level that it 

 is left exposed to the air for eight or nine hours at a time. The 

 apex of the shell of this species is nearly central, and the exterior 

 is sometimes nearly smooth, but more commonly relieved by radia- 

 ting ribs. 



Although the shell itself is not a particularly pretty object, it is 

 often rendered very beautiful and interesting by the various animal 

 and vegetable organisms that settle on it. Those shells that are 

 left dry for hours together are commonly adorned with clusters of 

 small acorn barnacles, while the limpets that have found a home 

 in a rock pool and are perpetually covered with water, often 

 resemble little moving gardens in which grow beautiful tufts of 

 corallines or other weeds, as well as polyzoa and other animal 

 forms. 



