MOLLUSKS 77 
move about, but as they become larger they lose this ability 
either wholly or periodically. In the edible mussels (AZyti- 
lus, Fig. 7), for example, which are associated in great 
numbers on the rocks along our coasts, the foot early be- 
comes long and slender and capable of reaching out a con- 
siderable distance from the shell to attach threads (byssus), 
which it spins, to foreign objects. These are remarkably 
strong, and when several have been spun it becomes a mat- 
ter of much difficulty to dislodge them. After remaining 
anchored in one situation for a while the mussel may vol- 
Fic. 47.—The edible mussel (Mytilus edulis), showing the threads by which it is 
attached. Natural size, from life. 
untarily free itself, and in a labored fashion move to some 
other more favorable spot where it again becomes attached, 
but there are numerous species, such as “fan shells” 
(Pinna), scallops, Anomea, and a few fresh-water forms, 
where the union is permanent. 
Finally, in the oysters, some of the scallops, and a num- 
ber of less familiar forms, the young in very early life drop 
down upon some foreign object to which the shell soon 
becomes firmly attached, and in this same spot they pass 
the remainder of their lives. The oyster usually falls upon 
the left half of its shell, which becomes deep and capacious 
enough to contain the body, while the smaller right valve 
