88 A NATURAL HISTORY OF THE SEAS 



and finest species are of tropical origin. Many close 

 their shells with a door or " operculum " of hard limey 

 substance, which is frequently of brilliant colouring. 

 The once popular cat's-eye brooches were derived from 

 a large Pacific species. 



The Periwinkles (Littorind) are in this country at least 

 of economic value and the largest examples are derived 

 from Scottish waters between tide marks. The winkles 

 are members of a group of molluscs which is gradually 

 exchanging a maritime for a terrestrial existence, one at 

 least of our native species never entering the sea, but de- 

 riving sufficient moisture from spray baths as it clings 

 on the cliff face well above high tide level. In conforma- 

 tion with its peculiar habitat the young are hatched within 

 the parent shell. 



The Cowry (Trinia europaed) is a member of another 

 familiar group famous for beauty of form and colour and 

 further distinguished for still being a recognised form of 

 currency in some parts of Africa. The animals are even 

 more resplendent than their shells and are of active 

 and predaceous proclivities. 



Nothing was known of the Cowry's metamorphosis 

 until 1926, when investigators in France and at Plymouth 

 discovered that the eggs of our native species are laid in 

 vase-like capsules embedded in an Ascidian on which this 

 particular Cowry feeds. 



The allied Poached-egg Shell (Ovula patula) of home 

 waters is invariably found attacking soft corals, eating 

 the polyps out of their protective coral cups with its long 

 pig-like snout. 



The Violet Snails {lanthinidae) come to our western shores 



