ITS MANNER OF LEAPING. 51 



"but I was still at a loss to know the modus operandi. 

 It appeared to me certain, that the ordinary supposition, 

 viz., that the action is performed by the vigorous 

 opening and shutting of the valves, was not the correct 

 one. At length a favourable obsei"vation gave me a 

 suspicion of the trutli. I perceived the lips of the 

 mantle, (which were held in contact, though the valves 

 were considerably separated,) suddenly open to a partial 

 extent, as if by a hlowimj from ivithin. At this 

 instant there w^as a leap in the opposite direction, 

 attended with a considerable agitation in the water. 

 With this clue, I observed more definitely. Having 

 rendered the water a little turbid, in order the more 

 distinctly to see any motion of the particles suspended 

 in it, several leaps confirmed the notion that had sug- 

 gested itself to me. The mode of proceeding is as 

 follows : when the Pecten is about to leap, it draws in 

 as much water as it can contain within the mantle, 

 while the lips are held firmly in contact. At this 

 instant the united edges of the lips are slightly drawn 

 inward, and this action gives sure w^aming of the com- 

 ing leap. The moment after this is observed, the 

 animal, doubtless by muscular contraction, exerts a 

 strong force upon the contained w^ater, wdiile it relaxes 

 the forced contact of the lips at any point of the cir- 

 cumference, according to its pleasure. The result is, 

 the forcible ejection of a jet of water, /)'cwi that point', 

 which, by the resilience of its impact upon the sur- 

 r{)unding fluid, throw^s the animal in the opposite 

 direction, with a force proportioned to that of they^^^ 

 dean. The action may be well imitated by the human 

 mouth l)l()wing a stream of air from any detemiined 



