727 
1888 - 89 .] Mr D. M‘Alpine on Bivalve Molluscs. 
and rotated through, a quarter of a circle on its posterior end in 
2J hours. A piece of the left outer gill rotated round the posterior 
end, whilst a second portion travelled about J inch in 3 hours. 
The sluggish movements of the entire gill in Unio contrast strongly 
with those in Mytilus. It may he that more extended observation 
will reveal a greater capacity for movement, but the gill certainly 
does not possess that readiness of movement characteristic of the 
sea-mussel. 
III. Mantle-Lobes. — Both right and left mantle-lobes were 
detached and laid down with their inner or ciliated surface upper- 
most, but no movement of any kind was detected. Then the right 
from another mussel was laid down with the outer or non-ciliated 
surface uppermost, and it moved away gently at once. It progressed 
in the direction of its cut surface, rotated around its posterior end 
(the opposite of the right outer gill), and travelled \ inch in 6 hours. 
Finely powdered charcoal placed upon the inner surface of the mantle 
was carried towards the free ventral margin and posteriorly. Two * 
small pieces were taken from the body of the mantle, the one {a) 
with its inner surface uppermost, the other ( b ) with its outer 
surface uppermost. The latter was taken from a thin part of 
the mantle, and moved ; while the former (a), which was thicker, 
did not move. 
The movement (b) was exceedingly slow, and could only be 
registered at distant intervals. In 15 J hours (at night) it had 
only covered of an inch, but in three hours during the day it 
travelled an equal distance. In 27 hours from the start it had 
progressed exactly half an inch altogether. After this it moved 
laterally as well as forward, so the exact distance traversed cannot 
be given. 
A strip of the muscular free margin, about one inch in length, 
was next detached from the posterior end. The two ends immediately 
came together, and a coil was formed, but there was no further 
movement ; while a similar piece from the anterior end remained 
perfectly still. 
The muscular margin was then cut up into very small pieces, 
about of an inch in length. These fragments were observed for 
several days, and only exhibited a very slight change of position 
from day to day. There was an evident absence of that motive- 
