280 



MANUAL OF ZOOLOGY 



developed eyes are present in a row round the edge of the 

 mouth in the scallop (Pecfen) and 

 a few others. 



Although none of the Pelecy- 

 poda are microscopic, they present 

 a considerable range in size, from 

 the little fresh-water Cyclas, about 

 i cm. long, to the giant clam (Tri- 

 dacna gigas} of the Indian and 

 Pacific Islands, which is sometimes 

 60 cm. (two feet) in length and 

 500 pounds in weight. The nacre- 

 ous inner layer of the shell of the 

 pearl oyster (Meleagrina marga- 

 ritifera), 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 between the mantle 

 and shell, or in the soft parts in 

 the pearl oyster and the pearl mus- 

 sel (Unio complanatus} , etc. 



Most Pelecypoda are sluggish 

 in habit, progressing only by slow 

 movements of the foot, and some 

 are permanently fixed during adult 

 life by the byssus. The scallops, 

 however, swim freely by clapping 

 the valves together. The cockles 

 (Cardtum), Trigonia, etc., jump 

 by sudden movements of the foot, 



ss 



FIG. 167. Teredo navalis, in 

 a piece of timber. P, pallets; 

 SS, siphons; T, tube; V, valve; 

 of shell. (From the Cambridge 

 Natural History.) 



