1888 .] 
Mr D. Alpine on Bivalve Molluscs. 
187 
began to separate at the posterior end, and gradually wrought itself 
off, until at last there was just a very narrow connection with the 
anterior end. The anterior end of the outer gill was now dragged 
round by the inner, so that, when they separated, the inner was 
perpendicular with its anterior end forward, while the outer was 
sloping towards it, with its anterior end backwards. The inner was 
now parallel with its original position, and away from it just the 
breadth of itself, or | inch. The outer soon became perpendicular 
too, and knocking against the other, which was now fixed at the 
edge of the plate, the course of both ended. Such an observation 
as the above shows that there is sufficient motive power in the inner 
gill to move away from the outer when placed upon it, also that the 
inner is the dominant gill (fig. 8). 
In both the inner and the outer gills the anterior end moves more 
quickly, the entire gill rotating on its posterior extremity. 
Portions op Gills. — If a small piece of inner or outer gill is 
taken and placed in liquid, it usually begins to move at once in 
the direction of the inner cut surface, presuming that the outer 
free margin forms the other boundary. It does not move straight 
forward, but usually to one side, and finally reaches the end of the 
plate with its cut edge, where it remains. If set adrift again, it 
moves about as before until it reaches the edge. It is also noted 
that the motive power in any piece of a given size is greater in the 
inner than in the outer gill, but that a strip of the margin does not 
move constantly, but commences to wriggle like a worm. It may 
bring the two ends together, then become straightened out ; but it 
is continually changing its position. 
Nature of Movement . — The movement is composite, consisting of 
translation and simultaneous rotation. It consists of a forward 
gliding movement in the direction of the cut surface, necessarily 
combined with a rotatory one. The rotation of the entire gill is 
almost always round the posterior end. 
Direction of Rotation of Portions of the Gills . — The result stated 
in the most general way is, that the gill in whole or in half turns 
in the same direction (upon posterior end), that extreme anterior 
and posterior pieces also turn similarly (upon posterior end), that 
the upper and lower corresponding pieces sometimes vary in their 
direction, and that lower pieces, unless at the extreme ends, in- 
