1888 - 89 .] Mr D. M ‘Alpine on Bivalve Molluscs. 
737 
muscle, is much broader than the remainder. The one end will be 
called the anterior and the other ' the posterior, for purpose of 
description. The mantle-lobe is attached during life to the inside 
of the valve. In the right mantle-lobe of the rock oyster there was 
not the slightest movement, whilst there was only slight movement 
of the two ends of the left when its lobe was laid down with its 
cut surface uppermost. It appeared as though the contraction of 
the centre brought about curling inwards of the two ends. 
Pieces of the mascular margin about a quarter of an inch in 
length can turn on themselves and change their position to a very 
slight extent. 
If the movements of the three forms, Mytilus, Unio, and Ostrea, 
be compared in their natural condition, Unio possesses the greatest 
activity, and Ostrea, as far as known, the least ; but if the progres- 
sive and rotatory movements due to cilia are in question, then 
Mytilus undoubtedly takes the lead. Unio, with its relatively large 
keel-shaped foot, can move along, either upright or on its side, in a 
way that neither Mytilus, with its byssus-secreting foot, nor Ostrea, 
without a foot at all, can approach ; but when the animal, divested 
of its shell, is placed in its native element, there is, perhaps, 
contrary to expectation, a power of progression and rotation in 
Mytilus only slightly shared by Unio, and almost absent from 
Ostrea. 
The general table will show at a glance the contrast between the 
nature, direction, and rate of movement in each of the various parts ; 
and it will likewise be noticed that each of the three forms has a 
distinct and specially active part, suggestive of underlying differ- 
ences. There is the gill in Mytilus, the ventral margin of the 
foot in Unio, and the labial palp in Ostrea. In Mytilus, no doubt, 
the palps are specially active, but they are eclipsed in rapidity and 
readiness of movement by the gills, so that the latter form the 
more prominent motile parts. 
Palps . — The palps have all a combined rotatory and progres- 
sive motion, normally, in the direction of the cut margin. As 
between Mytilus and Unio, the main difference lies in the two 
fellow palps on either side of the former, rotating in opposite direc- 
tions, and in the latter in the same direction. Ostrea may be said 
to agree with Mytilus, only the left outer or lowermost palp rotates 
