Tns FRESH-WATER CLAM 



235 



Fig. 226. — The scal- 

 lop (Pectcn irra- 

 dians). Right or 

 lower vah'o. One- 

 half nat. size. 



shaped shells attach in clusters or beds to rocks near low tide. 

 In the absence of rocks they may attach themselves to the 

 roots of the marsh-grass. They form so solid a wall here as 

 to protect the mud and the marsh-grass that 

 is growing in it from being washed away. 

 Thus the relations of marsh-grass and mus- 

 sels are reciprocally advantageous (Fig. 225) . 

 Mussels are etlible and in Europe thej^ are 

 reared for the market on woven nets, which 

 are submerged at each tide. 



The pearl mussels (Aviculidse ^) are a 

 group closely allied to the last, but not 

 represented on shores. They are important because they 

 produce the best pearls. Pearl-fishing has been carried 

 on since the earliest preserved historic records. The pearl 

 banlvs of Ceylon are known to have been fished for two 



thousand years. The finest pearls, 

 however, come from the Persian 

 Gulf. They are gathered there 

 by native divers, who, after tak- 

 ing several deep breaths, either 

 dive down unaided or descend 

 upon weighted ropes. When they 

 can no longer remain under water, 

 they either ascend again unaided 

 or are rapidly drawn up to the boats above. The pearl oysters 

 brought up to the surface are then examined for pearls. The 

 pearl is the secretion of the oyster about a small foreign body, 

 such as a grain of sand or a parasitic worm. This secretion 

 prevents the irritant from injuring the tissues. It is of the 



■ A small bird (avis) . 



Fig. 227. — One of the Ledida?, 

 Yoldia, foot protruding below 

 Nat. size. Photo, by W. H. 0. P 



