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 



developed. In a number jauttlw edulis, attached by its byssus (By) to a piece of 



of genera the foot contains wood: F : foot; s < excurrent si P hon - the branchial siphon 



being below it and not closed. 



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- 

 The 

 shows 



edulis, a common 

 east-coast pelecypod, at- 

 tached by its byssus to a 

 piece of wood. Most bi- 

 valves having a strong bys- 

 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. 



manent lodgment. 

 accompanying cut 



Ptcten opereularia, showing the ocelli, or eyes, on the 

 two edges of the mantle. 



