ACEPHALANS. 171 



among which we will only mention the oyster, clam, 

 mussel, and teredo. 



Oysters live in salt and brackish water, not far from 

 shore, and at but little depth. They nourish especially 

 in calm bays and the mouths of rivers. The often very 

 extensive colonies that they form on rocks are called 

 oyster-beds or banks, and from these they are taken by 

 the oyster fisherman by the aid of scoops or drags. 



The pearl oyster is celebrated for the pearly lining of 

 its shell and for the pearls found enclosed between the 

 shells. The pearls are produced by the ordinary shell- 

 excreting glands of the oyster, excited by the presence 

 of a foreign body, usually a grain of sand. Pearl-fishing is 

 carried on principally on the shores of Ceylon, but there 

 are also large beds of pearl oysters in the Persian Gulf, 

 in the Gulf of Mexico, and on the shores of Australia. 



Mussels are found on rocks near the shore. They are 

 much used as food, but some species app'ear to be poison- 

 ous. Some species inhabit fresh waters, both rivers and 

 ponds, and there is a marine species whose shell is prized 

 for its great lustre, and which often contains pearls. 



The teredos are, unfortunately, too well known by the 

 injuries they cause to wooden ships and naval construc- 

 tions. Several times they have nearly caused the sub- 

 mersion of Holland by perforating the dikes that pro- 

 tect that country from the sea. The teredo uses its shell 

 as an auger in driving forward its tunnel. 



The name molluscoid is applied to certain creatures 

 that are analogous to the acephalous mollusks ; they are 

 classified as tunicata and polyzoa. 



The tunicata have no shells, but the mantle that 

 envelops the body is thickened, sometimes being like 

 leather. They live in the sea, and have been divided 



