114 CLAMS, OYSTERS, AND MUSSELS 



sea water. It is necessary that they have plenty of fresh 

 sea water and food, and that the mud does not settle over 

 them, else they will smother. 



About Ceylon and AustraUa, in the Persian Gulf, around 

 the Philippines, Panama, West Indies, etc., is found the true 

 pearl oyster. The American form, which averages about 

 nine inches across, is somewhat smaller than the Old World 

 form, for the latter often becomes a foot in diameter. Pearl 

 fisheries are carried on in many different parts of the 

 world, but the most extensive ones are in Ceylon whUe the 

 finest pearls are said to be found in the Persian Gulf. 



The teredo, or ship worm, is a moUusk of strangely 

 modified form and interesting and unusual habits. It 

 bears very little resemblance to a bivalve mollusk, for its 

 body is long and wormlike in appearance, although it has 

 a Small bivalve shell at one end and two long siphons 

 at the other. The embryonic teredo is ciliated and free- 

 swimming like the oyster, but instead of settling on some 

 object and simply attaching itself, it finds a submerged 

 piece of wood, — for instance, the bottom, of a ship or piles 

 of a wharf, — and begins to burrow into this wood. As it 

 grows, it burrows deeper and deeper and lines its burrow 

 with a calcareous deposit. When a large number of these 

 small mollusks attack the piles of a wharf or the bottom 

 of a ship they fairly riddle them with holes a foot or more 

 in depth and cause much damage. 



Snails, Slugs, and Seashells 



Class II. — Gasteropoda (stomach-footed) 



To this class belong the snails, slugs, etc. The snails 

 have a shell composed of a single piece, while the slugs have 



