BIVALVE MOLLUSCANS. 265 



means by which they do this has not been 

 clearly explained they support themselves half 

 under water. They next open their shells, to 

 which they communicate such a vibration, that 

 they acquire a very brisk movement from right 

 to left, which enables them, as it were, to run 

 upon the water. 



The tulip-shell, 1 when it walks, if I may so 

 speak, opens and shuts its valves, and at the 

 same time lengthens and shortens its foot, 

 which seems to indicate a connection, or action, 

 between the former and the latter organs, 

 similar to what has been observed to take place 

 in insects, and perhaps points out some analogy 

 between the valves of the shell and the upper 

 wings, or elytra of insects, and the mantle and 

 their under wings. 



Bosc states, that the animals of the genus 

 Venus, in calm weather, may be seen sailing 

 on the surface of the waters, using one of their 

 valves as a boat and the other as a sail. As 

 these are usually rather heavy shells, they 

 must be furnished with some means of rendering 

 themselves lighter than the water. Pliny, of 

 old, mentions shells dedicated to Venus, which 

 sail and oppose their concave part to the wind. 



Thus we see the Creator has given even to 

 these apparently stupid and inactive creatures 

 means of enjoyment, that every one is not aware 



1 Tellina. 



