176 Proceedings of Royal Society of Edinburgh, [march 5 , 
usual way by inserting a knife at the ventral surface, and passing 
it round the posterior end until the posterior adductor muscle is 
cut through, then if the two valves are spread out, with their 
pointed ends directed forward, the right and left valves will lie 
just reversed from our own right and left. This is the position 
from which our observation will be made, and fig. 1 shows the four 
parts so arranged. 
In describing the rotation of the labial palps, it will be found 
very convenient to use the terms right-handed and left-handed, as 
is done in connection with the rotation of the plane of polarisa- 
tion. So when rotation occurs in the direction of the hands 
of a watch placed face upwards, i.e., from left to right, it will 
be called right-handed, and when in the opposite direction left- 
handed. 
The mussel, as seen in fig. 1, may be regarded as the watch, only 
the labial palps represent the hands as they are when half round, 
so this must be remembered in settling the direction of rotation. 
The labial palp will accordingly be spoken of as right-handed or 
left-handed according to its rotation ; and the direction of watch- 
hands, as seen by the observer, will be taken as the right-handed 
standard. 
The attachments and connections of the various parts will be 
briefly noted, in order to understand how they perform their 
functions, but minute structure will not be regarded, except in so 
far as it explains the movements. In most cases fresh mussels were 
taken, because the object of the inquiry was to find out how certain 
detached portions behaved under circumstances as near as possible 
to that under which they naturally existed, and fresh mussels were 
considered indis|)ensable. I even tried some experiments at the sea- 
side, taking the mussels direct from the sea, but equally good results 
were obtained after bringing them home. With regard to the 
temperature at which the observations were made, it ranged from 
13° to 19° C. The specimens were examined principally in August, 
September, and October, and no attempt was made to increase the 
speed by artificial heat, as might have been done, since the effect of 
such variations will be considered under the appropriate heading. 
The exact temperature is usually given in the tables, which is the 
temperature of the room. An increase from 19° to 28° C. is known 
