ventral margiu foremost. An inclination to one side or 

 tke 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 is, 

 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 is, 

 perhaps, a modification due to the development of the 

 power of swimming and we may also put down to this, the 

 evolution of 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 niaximus or P. 

 ojyercularis 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. maximus 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. I have not been able to 



