1888 - 89 .] Mr D. M‘ Alpine on Bivalve Molluscs. 
water mussel. There was rotation as well as progression in both, 
but if we compare the latter, then the mantle-lobe of Mytilus has 
the decided advantage. That of Mytilus went at the rate of 1 inch 
in 50 minutes, and that of Unio an inch in 24 hours. The foot 
of Mytilus is so entirely different from that of Unio that comparison 
will not be attempted. Suffice it to say, that the slow-moving, definite- 
shaped, steadily directed foot of Mytilus, when attached, is a perfect 
contrast to the same when detached, while the free margin of the 
foot of Unio appears to possess all the activity of the keel-shaped 
mass. 
The cilia of the sea-mussel are almost instantly arrested in their 
movements by fresh water, just as those of the fresh- water mussel 
are by salt water, but it is interesting] to note that each may be 
gradually accustomed to the changed conditions. Professor Semper, 
in his Animal Life , mentions the case of a Unio living within 
reach of the flood tide, of the sea-mussel in perfectly fresh water, 
and of the edible oyster in brackish water. This may help to 
explain how the cilia originally became gradually adapted to salt 
water in the one mussel, and fresh water in the other. 
There are still two important considerations suggested by the pre- 
ceding observations — the resting of the cilia and the relative and 
absolute rate of movement of detached parts. 
It is pointed out that the cilia are supposed to continue their 
work without any rest, and it may be conceded that microscopic 
examination goes to prove this, but from the observations made on 
the variation in direction of the movements of pieces of gills, &c., 
within very short periods, such can scarcely be the case ; and it 
must be argued that different series of cilia are brought into play 
at different times. It may be imagined, in a structure like the gill, 
with its innumerable cilia, that they rest in relays without inter- 
fering much, if at all, with the general effect ; and I have often had to 
point out, as in the rotating palps of the oyster, that in the course 
of long spells of rotation they “ rested half a minute ” or so. To 
the observer this looks exactly like “ stopping to take breath,” or 
the wearied rower laying down his oar for a minute. 
In the course of these investigations an important distinction was 
noticed between the action of the cilia and the movement of the 
cilia-bearing mass. The movement of the mass might cease, and 
