MOLLUSKS 138 
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 
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 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 1896 the pearls obtained from mussels in 
Arkansas were valued at 835,000, some of them being worth over 
$1,000 apiece 
The Seallop, (Peeten irradians, Fig. 95). The common scallop 

Fig. 95; 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 shallow 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 composed of stout, 
thread-like anchorages secreted by a gland in its foot. The little 
