PHYLUM MOLLUSCA 



279 



shell are extremely small, and the general surface of the 

 mantle secretes a continuous 

 shelly tube lining the burrow in 

 which the elongated worm-like 

 body of the Mollusc lies. 



Eyes are present in a row round 

 the edge of the mantle in the 

 Scallop (Pecteri) and a few others. 



Although none of the Pelecy- 

 poda are microscopic, they pre- 

 sent a considerable range in 

 size, from the little freshwater 

 Cyc/as, about i cm. long, to the 

 Giant Clam (Tridacne gigas) of 

 the Indian and Pacific Islands, 

 which is sometimes 60 cm. (two 

 feet) in length and 500 pounds 

 in weight. The nacreous inner 

 layer of the shell of the Pearl 

 Oyster (Meleagrina margariti- 

 fera\ which is of unusual thick- 

 ness, constitutes the " mother-of- 

 pearl " of commerce, employed 

 for many ornamental purposes. 

 Pearls are deposits of nacre 

 formed around sand-grains or 

 other foreign bodies, either be- 

 tween the mantle and shell or 

 in the soft parts, in the Pearl 

 Oyster and the Pearl Mussel 

 ( Unto margaritiferd). 



Most Pelecypoda are sluggish 

 in habit, progressing only by 



ss 



FIG. 155. -Teredo navalis, in 

 a piece of timber. SS, siphons ; 

 T. tube ; V. valve of shell. 

 (From the Cambridge Natural 

 History.) 



