SEA-FISHERIES LABORATORY. 339 
ventral margin foremost. An inclination to one side or 
the other can be effected by partial closure of one of these 
dorsal openings. 
The sudden retraction of the tentacles is always the 
signal for the closing of the shell. 
The animal can, in addition, force the water out at 
the ventral margin by not bringing the pallial barrier 
into play. This occurs when it is suddenly stimulated, 
and then it darts away with the hinge line foremost. It 
also is interesting to note that when the animal is turned 
over on to the upper side, it rights itself in a very short 
time by driving water out sharply between the ventral 
margins of the shell. This forces the hinge line back 
against the ground and is then used as a fulcrum on which 
to turn over. When in the normal position, that 1s, 
lying on the convex valve, a slight jet of water sent out 
ventrally causes that edge of the shell to rise from the 
bottom, so that the normal movements of swimming can 
take place without any hindrance from friction with the 
bottom. 
The equilateral character of the shell of Pecten 1s, 
perhaps, a modification due to the development of the 
power of swimming and we may also put down to this, the 
evolution cf a muscular velum, the large single adductor 
muscle with its adaptations for rapid contraction, and also 
the large internal cartilage for opening the shell. 
It is doubtful whether adult Pecten maaimus or P. 
opercularis ever employ the foot for purposes of locomo- 
tion. This seems to be rudimentary in the adult as far as 
its use as a locomotive organ is concerned, but as on one 
occasion I was able to see a P. maaimus protrude its foot 
—which is evidently capable of much distension—out of 
the shell, it may be possible that in its normal habitat it 
uses the foot more frequently. JI have not been able to 
