MOLLUSKS 



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Fig. t),s: COMiMDN SCALLOP. 



that waves through the water. If by chance this thread comes in 

 contact with the fins or scales of a fish it instanth' attaches itself, 

 and draws up the little mussel 

 so that it is enabled to snap its 

 shell upon the fin and hold tight- 

 ly by means of its sharp spines. 

 Tliis irritates the tissues of the 

 fish, so that the skin grows over 

 the little attaclied mollusk, en- 

 closing it in a capsule or cyst. It 

 remains tlius for from two weeks 

 to more than two months, and 

 finally frees itself from the fish 

 and drops to the bottom as a well 

 developed mussel. The mussels 

 are thus transported from stream to stream through the agency 

 of fish, and this accounts for their very wide distriljution. 



The fresh-water mussels of lakes and ponds are thin-shelled 

 and belong to a group called the anodontas, while those of running 

 streams are thick-shelled and are called unios. They grow very 

 slowly and do not begin to breed until they are from tliree to seven 

 years old, although tliey probably live to be from fifteen to twenty- 

 five years of age In 1S9G the pearls obtained from mussels in 

 Arkansas were valued at ij^ 05,000, some of them being worth over 

 $1,000 apiece 



The Scallop, fPecten irradians, Fig. 95 ). The common scallop 

 ranges from Tampa, Florida, to Nova >Scotia. It is most abundant 

 near the eastern end of Long Island Sound, and, while common at 

 Provincetown, Cape Cod, is exceedingly rare north of that place. 

 It lives best in shalloAv l)ays, and harbors, where the bottom is apt 

 to be sandy or covered with eel grass. The shell is flattened at the 

 hinge, forming a pair of "ears," and about 19 radiating ridges 

 extend outward from the beak of the shell. Professor Davenport 

 found that long ago in Pliocene times the scallops had from 19 to 

 22 of these ridges but that the normal number for modern shells is 

 only 19. When the scallop is young it attaches itself to eel grass, 

 or other submerged objects, by means of a byssus composed of stout, 

 thread-like anchorages secreted by a gland in its foot. The little 



