UNIVALVE MOLLUSCANS. 271 



sometimes conceal themselves in their fissures : 

 they often traverse vast tracts of ocean fixed to the 

 keels of ships, like some of the limpets they fix 

 themselves a good way out of the water, so as 

 only to be wetted when the tide is up, and some- 

 times above high water mark. Poli says that 

 when they resist any attempt to force them from 

 their station, they expel the air and water on all 

 sides and produce a vacuum, so that it is very 

 difficult to overcome the pressure of the atmos- 

 phere ; and Mr. Frembly, who had an oppor- 

 tunity of studying their habits on the coast of 

 Chili, states that when not apprehensive of danger 

 their attachment is very slight, and by pushing 

 them gently they will easily slide from the sur- 

 face to which they are attached, but if a direct 

 attempt is made to unfix them by force, they 

 will part with a portion of their shells sooner 

 than let go their hold. 



When we consider that these animals are not 

 only often exposed to the violent action of the 

 waves, but also to the attack of countless 

 enemies, we see abundant reason for the coat 

 of mail with which their Creator has covered 

 them. Even the fleshy or cartilaginous margin, 

 or zone, as my lamented friend the Rev. Lans- 

 down Guilding, in his admirable memoir on this 

 tribe, denominated it, is defended sometimes by 

 scales, spines, and bristles, at others rough with 

 numerous little bony tubercles ; it is also de- 



