PELECYPODA 



77 



Oysters abound in quiet, shallow inlets of the Atlantic coast south of 

 Cape Cod, and of the Gulf of Mexico. We have the best oysters in the 

 world. ^ Our most extensive oyster-beds are on the Chesapeake Bay, at 

 Baltimore, where they cover 3000 acres and furnish millions of bushels 

 yearly. We not only supply the markets of our own great cities, but send 

 large quantities to British markets. Oysters are found also on the Pacific 

 coast, on the coasts of Europe, of Australia, 

 and of Japan. 



The scallop {Pecten) has an almost round, 

 fluted shell with a straight hinge without 

 teeth, and with unequal valves, one being 

 more nearly flat than the other. The shell is 

 usually brilliantly tinted. The foot is rudi- 

 mentary or altogether lacking. The mantle- 

 folds are fringed with slender tentacles and 

 the edge of each lobe is set with a row of 

 brilliant bluish " eyes." When at rest the 

 scallop lies on the sea bottom with its one ad- 

 ductor muscle relaxed and its shell open. If 

 disturbed, it quickly closes tlie shell by con- 

 tracting the strong muscle. This catches a 

 quantity of water which is forcibly ejected 

 through a round aperture at either end of the 

 straight flange of the hinge. The reaction 

 caused by forcing this water against the great 

 body of water outside propels the animal for- 

 ward. Thus, by rapidly opening and closing 

 its shell, it swims through the water with 

 comparative ease. 



The edible scallop (Pec^ten irra'dians) is 

 about 2 J inches in diameter and its color 

 varies from a whitish to a reddish or purple 

 hue. The adductor muscle is the portion used by man for food. This 

 scallop is found on the Atlantic coast south of Cape Cod. 



Pec' ten max'imus, found on the coast of Great Britain, in water 30 to 40 

 fathoms deep, is much larger. Its deeper shell was formerly used as a 

 baking-dish for oysters, hence the origin of the term " scalloped oysters." 



The shell of another form common in the Mediterrnaean Sea {Pec'ten 

 jacohae'us) was worn as a badge by the crusaders returning from the Holy 

 Land. 



The so-called pearl-oyster (M,l,(i<jn'na), which does not belong to the 

 oyster family at all, has a shell whicli is more nearly circular, a little convex, 

 and sometimes a foot in diameter. They are found in Madagascar, 

 Panama, Ceylon, East Indies, Australia, South Sea islands, PhiHppines, 

 and the West Indies. 



Pearls are deposits of nacre formed about some foreign substance. 

 Prof. Jameson has discovered- by investigation upon the sea-mussel that, 

 in their case, pearls are caused by a parasitic worm {Trematode) . Pearls 

 are collected by divers who go down from 6 to 8 fathoms for them. Hun- 



1 "On the coasts of Holland, Belgium, and France far greater care is taken 

 of their species [Os'treaed'tdis) than we take of ours {Os'trea Virginia' no) , 

 but our natural conditions are superior to theirs." — Linville and Kelly, p. 169. 



- Linville and Kelly's " General Zoology," p. 173. 



Fig. 56. — Right side of 

 embryonic oyster, six days 

 old: m, Mouth; s, vent; /, 

 right lobe of liver; vl, 

 velum. (Moore, Bull. U. 

 S. F. C, 1897.) 



