MOLLUSKS 



269 



source of our food supply. Many of the mollusks also make up 

 an important part of the food supply of bottom-feeding fishes. 

 On the other hand, 

 some mollusks, as A^a- 

 tica, bore into other 

 mollusk shells and eat 

 the animal thus at- 

 tached. Some boring 

 mollusks, for example 

 the ship worm ( Teredo 

 navalis), do much 

 damage, especially to 

 wharves, as they make 

 their home in piles. 

 Still others bore holes 

 in soft rock and live 

 there. 



The shells of mollusks are used to a large extent in manufacture 

 and in the arts, while they form a money basis still in parts of 

 the world. Sepia comes from a cuttlefish. 



Pearls and Pearl Formation. — Pearls are prized the world over. It is 

 a well-known fact that even in this country pearls of some value are some- 

 times found within the shells of such common bivalves as the fresh-water 

 mussel or oyster. Most of the finest, however, come from the waters around 

 Ceylon. If a pearl is cut open and examined carefully, it is found to be 

 a deposit of the mother-of-pearl layer of the shell around some central 

 structure. It has been believed that any foreign substance, as a grain of 

 sand, might irritate the mantle at a given point, thus stimulating it to 

 secrete around the substance. It now seems likely that perfect pearls are 

 due to the growth within the mantle of the clam or 03^stcr of certain para- 

 sites, stages in the development of a fluke worm. The irritation thus set 

 up in the tissue causes mother-of-pearl to be deposited around the source 

 of irritation, with the subsequent formation of a pearl. 



Piece of timber, showing holes bored by the ship worm. 



Classification op Molt.ttsks 



Class I. Pelecypoda (Lamellibranchiata). Soft-bodied unsegmented animals 

 showing bilateral symmetry. Bivalve shell, platelikc gills. Examples, 

 clam (Mya arenaria), scallop ipecten), oyster (Ostrea), and fresh-water mussel 

 {JJnio), 



