1888.] Mr D. M‘ Alpine on Bivalve Molluscs. 177 
to render the movement of cilia six times as fast, and so increase of 
temperature will alter the rate of motion considerably. 
It may be useful to mention, once for all, that for ordinary pur- 
poses of observation T found nothing better than an upturned plate 
whereon to lay the object. The rim of the bottom just served to 
keep the fluid in, and enough was usually obtained from a shell, 
while the potter’s marks upon it served to indicate the smallest 
amount of movement. Each dot, line, figure, letter, or other device 
was a handy guide, and in the plates I used there happened to be a 
rhomboid figure, exactly 1 inch in length, which just suited my 
purpose. 
When plenty of room was “required, as, for instance, to allow the 
gill free scope in its movements and rotations, the largest dish pro- 
curable was used, and there, too, the leaves and lines were excellent 
guides. 
As I used plates of the same size and pattern, it was easy, when 
found necessary, to trace the course of any of the parts with pen and 
ink on an empty plate, — say the track of a rotating gill or palp, or 
of an advancing foot, and transfer it direct to paper. 
The gills are very delicate structures and readily break up, but if 
sea-water is placed in the valve of the shell, so as to keep them 
afloat while being detached with a fine pair of scissors, they may be 
transferred to the plate entire. The parts are laid out with their 
inner surface uppermost, as they lie in fig. 1, and this is the position 
of movement recorded, unless specially stated otherwise. 
Section A. — Entire Animal removed from Shell. — A small 
mussel, when carefully removed from its shell and immersed in a 
vessel of sea-water, exhibited a rotatory movement with slight for- 
ward movement, the posterior end sweeping round whilst the 
anterior extremity formed a sort of movable pivot. The two ends 
moved round in opposite directions, the anterior being the point 
of least motion. This motion was continuous, and always in the 
initial direction. The movement of the foot was quite independent 
and irregular, in one case the direction being right-handed, in 
another left. After several trials, some light was thrown upon this 
change of direction. It is difficult to lay out specimens quite 
evenly, so that the body-axis will be exactly central and the lateral 
members equally disposed on each side. I observed that in sped- 
