738 Proceedings of Royal Society of Edinburgh. [sess. 
like its fellow, or it may be said to agree with Unio, in the palps of 
both sides rotating alike, only the right outer or uppermost can 
turn either way, by preference, the opposite to that of Unio. The 
rate on the whole is in favour of the oyster, the general average 
being 3J- minutes per round ; next comes Mytilus, with a general 
average of 4 ; and lastly Unio, with a general average of 8J. It will 
be remarked that the average of the corresponding palps agree 
pretty closely in Ostrea and Mytilus, and that it is the left outer 
which is the slowest in both. 
Gills . — As regards the movement of the gills in their entirety, . 
there is hardly any comparison between those of Mytilus and the 
other two. All agree indeed in moving, and in moving in the 
direction of the cut margin, but beyond that they have little in 
common. The noticeable feature in the gill of Mytilus is its 
readiness, when detached and laid out, to move away ; and so 
energetic is it that even the anatomists have not failed to notice 
the swimming movement of parts of it, and as far as known to me,, 
this is the only mollusc in which such has been noticed. It is the 
only part of any mollusc examined so far which can be absolutely 
relied upon to move steadily and immediately when detached, 
hence its great suitability for investigations concerning cilia and 
their movements. 
The palps as a rule could be trusted to rotate after a longer or 
shorter resting period; but it was noticeable in the oyster, that of 
the two fellow palps detached from either side, only one of them 
usually rotated, so much so, that after spending much time at first in 
detaching single specimens, I got into the habit latterly of detaching 
and laying out two together, with the sure and certain hope that 
one of them, at least, would not disappoint me. 
The average rate of movement for the gills of the sea-mussel is 
2 minutes per inch for those of the fresh-water mussel ; there is no 
general average, but -J- inch done in 3 hours; and for those of the 
oyster the average of about 13 minutes per of an inch, or 85 
minutes for one measured inch; or, to express it in a comparative 
way, 1 inch is traversed by the gill of Mytilis, Ostrea, and Unio in 
2 minutes, lj hours, and 24 hours respectively. 
As there was no definite progressive movement of the entire lobe 
observed in the oyster, the comparison lies between the sea and fresh- 
