194 



Shell Life 



excretory orifice, corresponding witli an oval tube 

 in the mantle. The animal has two large gills, 

 placed one on each side. It is probable that from 

 some such form the bivalves were produced, the 

 development of these organs into the plate-like gills 

 producing corresponding changes in the mantle, and 

 consequently of the shell. 



The Keyhole Limpet (Fissurella grceca) in the 

 juvenile stage has a distinctly spiral beak, and a 

 slit margin, but as the shell grows, and the em- 

 bryonic portion becomes the apex, the slit finall}' 

 appears like a keyhole at the top 

 of the shell, and the niantle is 

 protruded through it. It is a 

 pale yellowish shell, with rays 

 of some tint of brown, and about 

 50 rugged ribs radiating from 

 the beak, which are alternately 

 larger and smaller. The animal is some shade of 

 yellow ; the mantle extends beyond the margin of 

 the shell, and is fringed with short stout processes 

 corresponding with the ribs of the shell which are 

 produced by them. A similar fringe decorates the 

 upper part of the foot. The Keyhole Limpet is 

 found all round the British Islands with 

 the exception of tlie east of England 

 and the east and north of Scotland. It 

 extends from low water to a depth of 

 15 fathoms. The Punctured Limpet 

 (PiinctureUa noachina) is much smaller than the 

 last, but agrees with it in having a slit, which does 

 not, however, take the place of the beak, but is a 

 little in front of it, as shown in annexed view from 



Keyhole Limpet 



