PELECYPODS 415 
THE FOOT 
The foot undergoes great variation, from a strong, powerful 
organ capable of forcing its way through heavy gravelly bottom 
or burrowing deeply into 
the sand or mud, to the 
merest trace of a pedal or- 
gan. In the oyster the 
foot has become almost en- 
tirely atrophied; the sta- 
tionary life led by the ani- 
mal renders such an organ ; 
quite unnecessary. Ex-~ 
cepting in a few families, 
the pelecypod foot is well _ Bi Zz 
ss 7 
developed. Ina number Mytilus edulis, attached by its byssus (By) to a piece of 
of generathe foot contains ine dhs me ee siphon, the branchial siphon 
a gland for the secretion 
of long silk-like or horn-like fibers, which are collectively called 
a byssus. The use of a byssus is for attachment to any object to 
effect a temporary or per- 
manent lodgment. The 
accompanying cut shows 
Mytilus edulis, a common 
east-coast pelecypod,  at- 
tachéd by its byssus to a 
piece of wood. Most bi- 
Pecten opercularis, showing the ocelli, or eyes, on the valves having a strong bys- 
eager eaters sus exhibit a feeble develop- 
ment of the foot; nearly all bivalves, however, show traces, some- 
times only in the embryo, of a byssal gland. 
The visceral mass contains the liver, the exceedingly large gen- 
erative glands, the kidneys, etc., and through all this soft whitish or 
reddish mass the alimentary canal wanders about in a tortuous 
fashion, finally passing through the pericardium and the ventricle 
of the heart, and terminating just over and back of the posterior 
adductor muscle. 
