736 Proceedings of Royal Society of Edinburgh. [sess. 
observed to send materials towards the mouth end when detached, 
it is rendered extremely probable that the palps in the oyster 
exercise the double function of guarding the mouth, and of guiding 
food materials towards it, at the proper time. 
Right Inner Gill-Plate. — In the rock oyster the entire gill exhibited 
only faint indication of movement in the direction of its cut 
surface. Pieces of the gill-plate of the mud oyster laid on the 
inner surface moved forward and to the left, then one began to 
rotate on its anterior end, then the posterior end became the pivot, 
and in 57 minutes it had returned to its original transverse position, 
but to the left and about one inch higher up. Placed in its 
original position again, the movements were exactly reversed. It 
retained very slight power of movement for five days. 
Right Outer Gill-Plate — (a) Entire rock oyster. — The move- 
ments were very irregular and slight, the only definite result being 
that the gill-plate moved as a whole ^ inch in 6 hours. ( b ) Pieces 
of the corresponding gill-plate of the mud oyster exhibited no 
movement of any kind. 
Left Inner Gill-Plate — (a) Entire rock oyster. — One specimen 
moved in the direction of the cut surface, moving more at the ends 
than at the centre. It thus moved forward in this irregular 
manner Iff of an inch in 4 hours 50 minutes. The free edge 
travelled Iff inch, whilst the cut edge had worked through only If 
inch during the same period, (b) Pieces (of mud oyster) gave 
absolutely negative results. 
Left Outer Gill-Plate — (a) Entire rock oyster. — Beyond slight 
movement in the direction of the cut surface, there was no change 
in position. ( b ) Pieces (mud oyster). A small piece half an inch 
in length, completed a revolution in 3 hours 1 2 minutes, and moved 
slightly forward as a whole ; the third and fourth quarter revolu- 
tions were performed in 30 and 32 minutes respectively. 
Mantle-Lobes. — Each mantle-lobe of the oyster contracts consider- 
ably on being separated from the shell, hence on the opened side 
it is greatly shrunken, but it may be removed from the remaining 
valve in a fair condition for laying out. Thus the mantle-lobe 
when detached has not the compact form of that of Mytilus, but 
is elongated, curved, more or less puckered, and the portion 
extending from the mouth end, and sweeping round the adductor 
