MOLLUSKS 



77 



move about, but as they become larger they lose this ability 

 either wholly or periodically. In the edible mussels (Myti- 

 lus, Fig. 47), 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- 



Fio. 47. The edible mussel (MytUus 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, Anomia, 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 



