1 46 DIVERSIOxNS OF A NATURALIST 



(one species of which actually lives above high tide- 

 mark), the limpet, and the dog-whelk. A small species 

 of top-shell or trochus is also very common, and so is 

 the chiton, or armadillo-shell, which, though really the 

 most primitive and nearest representative of the ancestors 

 of all univalve molluscs, yet has its own shell of a very 

 peculiar character (sometimes with very minute eyes — 

 true eyes — dotted about on it), and always divided trans- 

 versely to its length (not right and left) into eight separate 

 pieces, which, indeed, seem to be really separate, in- 

 dependent little shells, corresponding to eight segments 

 like the segments of a shrimp or an earth-worm. 



Let us now compare the soft animal of one of the 

 bivalves — say the common cockle — with the soft animal 

 to which a univalve shell belongs — say the limpet. They 

 can be kept alive and watched in a finger-glass of sea- 

 water, and can be removed from their shells and examined 

 more closely — by killing them by dipping them for half 

 a minute into very hot (not boiling) water. Both these 

 molluscs — like all others — adhere tightly at one place to 

 the shell. They cannot be removed from it alive, and 

 make a new shell or creep back into the old one, as can 

 some worms {e.g. the serpula) and other creatures which 

 form a hard shell to live in. Certain muscles of the soft 

 mollusc are so closely fixed to the shell that they must 

 be torn in order to separate it. These muscles draw 

 the two valves of the bivalve together, and shut it tight. 

 You can verify this whenever the oyster-man u opens " 

 an oyster for you. When at rest the shells gape, being 

 kept open by the horny, elastic hinge-piece. Some 

 bivalves (for instance, the common scallop, or pilgrim's 

 shell, which can often be dredged in shallow water, and 

 of which a large kind is sold in the London fish shops) 

 actually swim in the sea-water by aid of this mechanism, 



