. A manual of zoology. 280 MANUAL OF ZOOLOGY developed eyes are present in a row round the edge of the mouth in the scallop (Pecten) 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 (Afeleagrina marga- ritiferd), which is of unusual thick- ness, constitutes the " mother-of
. A manual of zoology. 280 MANUAL OF ZOOLOGY developed eyes are present in a row round the edge of the mouth in the scallop (Pecten) 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 (Afeleagrina marga- ritiferd), 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 Fig. 167. —Teredo navaiis, in the valves together. The cockles a piece of timber. P, pallets; / *-i r> \ rri • SS, siphons; ; V, valve; (tari7UI7/l) , 1 ngonid, etc., JUlTip of shell. (From the Cambridge 1 11 c ^ c . Natural History.) by sudden movements of the foot,. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Parker, T. Jeffery (Thomas Jeffery), 1850-1897; Haswell, William A. (William Aitcheson), 1854-1925. New York, The Macmillan Company; London, Macmillan & Co. ,Ltd.
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