ENDURANCE OF MOLLUSCS. 355 



submitting the animal to decomposition^ I found the 

 shell remain behind intact. 



Apropos of Molluscs : their powers of endurance are 

 very remarkable. Having noticed that they live out of 

 their native element, the water, for a considerable time, 

 being often left bare on the rocks by receding tides, I 

 thought of testing their powers in this way. Accord- 

 ingly, a Cockle was placed on my work-table, out of all 

 reach of damp, in a room where a fire was constantly 

 burning. This was on the 1 0th of April ; not until the 

 21st was the cockle dead. A small fish (Ophtdiuni) 

 under similar circumstances died in seven hours. 

 Whence this remarkable difierence in two gill-breath- 

 ing animals ? A question easily asked, but not easily 

 answered. 



It is true that both animals are aquatic, and both 

 breathe by gills ; but when we come to understand the 

 complex mechanism of respiration, we see various 

 special differences between the two organisms. Let us 

 begin with that of the fish. M. Flourens* has shown 

 that the weight of the soft leaflets composing the fish's 

 gill difi'ers but slightly from that of water ; so that 

 when the animal is in water the slightest force suffices 

 to float and separate them, by which means the water 

 bathes their surfaces, and the exchange of gases takes 

 place. But no sooner is the fish brought out of the 

 water than the difi'erence between the weight of its gills 

 and that of the atmosphere, immediately causes a col- 



* Experiences siir le Mecanisme de la Resp. des Poissons ; in the 

 Annales de Soc. Nat.y 1830, p. 5. 



