MOLLUSKS 133 



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

 contact with the fins or scales of a fish it instantly 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. 

 This irritates the tissues of the 

 fish, so that the skin grows over 

 the little attached mollusk, en- 

 closing it in a capsule or cyst. It 

 remains thus for from two weeks 



to more than two months, and ^^^^ W- ^ 



finally frees itself from the fish 

 and drops to the bottom as a well 



111 1 mi 1 Fig. 95; COMMON SCALLOP. 



developed mussel. The mussels 



are thus transported from stream to stream through the agency 



of fish, and this accounts for their A-ery wide distribution. 



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 three to seven 

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

 five years of age In 189G the pearls obtained from mussels in 

 Arkansas were valued at 835,000, some of them being worth over 

 '$1,000 apiece 



The Scallop, (Fecten 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, Avhile common at 

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

 It lives best in shalloAv bays, 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 comjDosed of stout, 

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



