1888.] 
Mr D. M‘ Alpine on Bivalve Molluscs. 
191 
The brown margin of the mantle-lobe and the whitish muscular 
margin both move about briskly, especially small pieces taken from 
the latter. The movement continues for several days, and in one 
instance it continued for a period of eight days, the rate of move- 
ment being about 2 inches per minute. 
Expla7iation of Duection of Movement. — The strong marginal 
backward current will tend to drive the mantle-lobe round at its 
anterior end, and the surface current from the attached or cut 
margin will drive it inward, and the result of these combined 
currents will be to cause the gill to rotate in a right-handed direc- 
tion. The margin of the mantle is a heavy muscular mass, particu- 
larly towards its posterior end, forming a constant drag, otherwise 
there might be considerable forward movement. Hence, I take it, 
the almost purely rotatory movement in striking contrast to the 
composite movement of the gill. 
The Foot is richly ciliated, and there is a slight notch at the free 
end, usually making the top slightly bifid, from which passes a thin 
white line (the byssal grove) to the posterior extremity. This 
groove may be converted into a closed canal by the meeting of the 
muscular sides. 
Movement — a, of entire free Portion. — If the free portion of the 
foot is detached and laid in water sufficient to cover it, a movement 
will take place in the direction of the tip or away from the cut 
surface. In all the parts hitherto tried the translatory movement 
Avas in the direction of the cut surface. 
The movement is usually in a direct straight line away from the 
cut surface, but the foot made a complete rotation in 6 hours 47 
minutes, and during this rotation the tip would be occasionally raised 
and swept round, or it might oscillate to and fro. The forward 
motion is at an average rate of I inch per hour, the highest speed 
attained being 1 inch in 24 minutes, the lowest 1 inch in 3 hours 
4 minutes. This is when moving on the ventral surface, but when 
moving on the dorsal surface the speed is only about one-third as 
great. The direction of rotation differs in the different specimens. 
Sometimes there is no movement at all, the specimen simply curling 
itself up and refusing to move. 
h, of Tip. — If a small portion of the tip is snipped off it moves 
freely to and fro, and is very sensitive to contact of any kind. 
VOL. XV. 2/10/88 N 
