THE LIFE-HISTORY AND HABITS OF THE PEAK!- OYSTER. 133 



and the remainder had thrown ott' their old hvssus and were preparing to attach. 

 probably waiting for the observers candle to be extinguished. 



The next morning all had attached. Several had crawled very long distances; one 

 had gone nearly to the opposite side of the tank, l'7 inches (the greatest distance 

 we have seen covered in the J "J hours), another had travelled 8 inches, and a third. 

 L3 inches. This rapidity of re-attachment under healthy conditions (one within 

 - hours, the majority within 8, and all within 20 hours) has obviously an important 

 hearing upon the proposed transplantation operations on the pearl hanks. 



Pearl oysters are extremely sensitive to light when preparing to attach, and there 

 is always difficulty in seeing the actual operation of byssal fixation. While watching 

 the oysters in one tank, those in another will he fixing, and when the light is brought 

 to bear upon these last, they will withdraw the foot as soon as the strand they are 

 engaged upon is finished, and refuse to again protrude until the light is removed. 



When the pearl oyster is crawling, while advancing the foot the valves are widely 

 open. When fully protruded the animal contracts the toot to the utmost, thus 

 dragging forward the body. At the same time the valves are usually brought 

 together with a snap, whereby the advance is materially aided. 



Our observations show that the disc of attachment of a byssal thread is not formed, 

 as Kelaart supposed, by a fosset-like expansion of the pedal groove at the tip of the 

 foot. In reality the pedal groove subserves two distinct functions, the anterior or 

 distal part being used wholly for locomotion and having nothing to do with hvssus 

 formation, which is effected by the hinder region of the groove alone. The front end 

 of this hinder section, at its junction with the locomotor surface and about midway 

 between the pedal tip and the anterior margin of the byssal pouch, expands to form 

 an oblong cup-shaped pit or sucker. Behind this again the margins of the groove are 

 usually curved inwards, meeting medianly, to form an extemporised laterally compressed 

 cavity, the pedo-byssal tube : anteriorly this communicates with the median sucker-pit 

 and posteriorly with the byssal pouch. 



When the foot is protruded for the purpose of forming a byssal thread, the fore part 

 of the pedal groove is flattened out to act as a sucker, holding the loot in position. 

 The lips of the median pit also expand, forming an oval sucker attachment, while the 

 edges of the hinder part of the pedal surface remain approximated, and may not even 

 touch the surface to which attachment is to be made. For about 4 to 6 minutes, the 

 oyster remains at rest in this position, while a secretion is flowing actively from the 

 byssogenic glands into the extemporised tube along the hinder part of the foot, and so 

 into the median sucker-pit. The latter is the only spot at which the secretion is in 

 contact with the exterior. At the end of the time mentioned the distal part of the 

 pedal surface frees itself, the margins of the proximal tubular region open, and the 

 foot as a whole is gently withdrawn within the shell, leaving behind a pale, glassy- 

 looking, laterally compressed thread attached distallv to the extraneous body by an 

 oval disc, ami at the inner end t<> the base of the byssal pouch. The oval attachment- 



