114 CLAMS, OYSTERS, AND MUSSELS 



sea water. It is necessaiy 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 .^ustialia, m the I'ersian Gulf, around 

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

 pearl oyster. The American form, which averages al)<jut 

 nine inches across, is somewhat smaller than the Old World 

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

 fisheries ai'e carried on in many different parts of the 

 world, hut the most extensive ones ai'e in Ceylon while the 

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



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

 modified form and interesting and unusual hal>its. It 

 bears very little resemblance to a liivalve 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 ami free- 

 swimming like the oyster, luit instead of settling on some 

 object and simply attadiing itself, it finds a suljmerged 

 piece of w(jod, — for instance, the bottom of a ship or piles 

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

 grows, it burrows deeper and deeper and lines its luirrow 

 with a calcareous deposit. When a large nundier 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. 



Sn.'Vils, Slugs, and Se.vshells 



Class II. — Gnsleropoda {stomar]> -footed) 



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

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



