338 SEA-SIDE STUDIES. 



where a fire was constantly burning. This was on the 10th 

 of April ; not nntil the 21st was the cockle dead. A small 

 fish (Ophidium) under similar circumstances died in seven 

 hours. Whence this remarkable difierence in two gill- 

 breathing 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 differs 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 

 brouQ-ht out of the water than the difference between the 

 weight of its gills and that of the atmosphere, immediately 

 causes a collapse of the former : the leaflets, instead of 

 floating free in the air, are pressed together, so that only the 

 external surface of the two outer leaflets are in contact with 

 the air, and this is obviously too small a surface to suffice 

 for the whole aeration of the blood ; and the fish dies of 

 asphyxia. Add to this cause, the rapid dessication which 

 ensues on exposure to the air, and which we know is an 

 obstacle to respiration. 



The Mollusc — our Cockle for instance — is somewhat differ- 



* Experiences svr le Mccanisme dc la licsp. ilcs Poi^suns ; in the -1 nnales des 

 Soc. Nat., 1830, p. 5. 



