136 BIVALVULAR LEAPERS. 



to the point of fixture. Such is the manner in which I have 

 seen the Cyclas, an inhabitant of our ponds, and some of 

 the lesser Bivalves which inhabit our shores, move along ; 

 and, I presume, it is in a similar manner that the other and 

 larger species proceed. Reaumur has happily compared 

 their mode of progression to that of a man who, having laid 

 himself flat on his belly, desires to move onward by the sole 

 aid of his arms : he stretches the arm to a point of support 

 which he can just reach with the hand, and take hold of, 

 when, by shortening the arm, he drags his body on ; and the 

 foot of the Bivalves differs from the arm only in this, that 

 the shortening is effected by a general contraction of the 

 muscular fibre, and not by muscles bending a joint.* 

 Reaumur also informs us, that some Bivalves, as the Myae, 

 can move retrogradely along the ground in this manner. 

 They plant the point of the foot just beyond the margin of 

 the valves in the clay or mud, and then by elongating the 

 foot, they push themselves backwards, in the same way that 

 a sailor pushes off a boat by leaning against the oar which 

 he has planted in the sand on one side. 



When the bend in the foot of a Bivalve is considerable, 

 forming a sort of elbow, the animal is projected forward 

 by a succession of short leaps. Such a structure charac- 

 terises the Tellinae and Donacidae, and you may see it well 

 marked in the Donax trunculus of British authors, a species 

 which is abundant on most of our sandy shores. When it 

 is about to make a spring, it firstly, by appropriate motions 

 of the foot, puts the shell on the point or summit, as if 

 aware that this is the position the most favourable of any to 

 avoid the resistance which the sand opposes to the motion. 

 It then stretches out the leg as far as possible, makes it 

 embrace a portion of the shell, and, by a sudden movement, 

 similar to that of a spring let loose, it strikes the earth with 

 its leg, and effects the leap.f This, in some species, is 

 considerable, for Mr. Sutchbury told the Rev. Mr. Kirby 

 that Trigoniae of New Holland would leap over the gunwale 

 of a boat, to the height of above four inches. £ Some Clams 

 or Pectens, and the closely allied Lima?, are also salient ; 

 but their leaps are the result of a sudden strong effort to 

 close the valves, after they have been opened to the ut- 

 most^ and which the remarkable size of the adductor 



* Mem. de l'Acad. Roy. des Sc. an. 1710, p. 581. 



t Smellie's Phil. Nat. Hist. i. 138 ; but more particularly Reaumur's 

 memoir just quoted, p. 600. 



t Bridgew. Treat, i. 264. 



§ Of Pecten islandicus Fabricius says, " Editur quidem ab incolis, sed 

 raro. Difriculter etiam coquitur: cum testa enim vivus vasi coquinario 



