MOLLUSC A, THE SHELL-FISH 109 



or muscular margin of the body. It contracts 

 this suddenly, after first opening the shell and 

 taking in as much water as possible. Thus the 

 water is squeezed out again, and the effect of 

 this is to propel the animal in an opposite 

 direction. 



Now we are in a position to understand a 

 little more about the shape of the shell. These 

 curious "ears" possessed by the two valves, to- 

 gether form a straight, strong edge, which cuts 

 the water as the animal flies along. It reminds 

 us of a ship's prow, and not without reason, 

 for the use of each is the same. A boat's sharp 

 prow, compared with the rounded front of a " tub," 

 makes all the difference in the possibilities of 

 straight steering, and favours the putting on of 

 speed : the ears of the shell are not less useful 

 to our Scallop. The following account of the 

 swimming powers of this species of Scallop, quoted 

 by Woodward, was given by the Rev. D. Lands- 

 borough, who observed young specimens, about 

 the size of the small ones in our picture, swim- 

 ming about in a pool of sea-water, left by the 

 ebbing tide. " Their motion was rapid and zig- 

 zag; they seemed, by the sudden opening and 

 shutting of their valves, to have the power of 

 darting like an arrow through the water. One 

 jerk carried them some yards, and then by 

 another sudden jerk they were off in a moment 

 on a different tack." To the sharp prow, the 

 Pecten owes this capability of arrow-like flight. 

 Its eyes are situated on the fringe of its mantle, 

 and consequently near the wide end of the shell ; 

 its peculiar mode of progression, therefore, en- 

 ables it to back away instantly from any enemy 

 it sees. 



