1 42 SCIENCE FROM AN EASY CHAIR 



cloud produced by them in the pools is of a deep cobalt- 

 blue. When oysters are placed in these tanks to fatten, 

 their gills or beards become rich blue-green in colour. 

 They lose the colour after ten days, when removed to 

 ordinary tanks. These are the celebrated green oysters 

 or " Marennes vertes " of French restaurants. The 

 colouring matter of the little diatoms swallowed by 

 the million and digested is taken up by the blood of 

 the oyster from its stomach, and is excreted by certain 

 corpuscles on the surface of the gills as I showed some 

 twenty-five years ago just as red madder is deposited 

 in the bones of a pig fed upon madder, and as the 

 feathers of the canary take up the colour of cayenne 

 pepper when it is mixed with the canary's food. It used 

 to be thought that the green colour of the green oyster 

 is due to copper and that opinion was supported by 

 the curious fact that the blood of all oysters and other 

 molluscs, and also of lobsters, scorpions, and king-crabs, 

 does really contain a minute quantity of copper, just as 

 our blood contains iron ! It was also supported by the 

 fact that occasionally a fraudulent fishmonger, when 

 asked to supply green oysters, has been convicted of 

 colouring the beards of ordinary oysters with green 

 copper salt, so as to imitate the real article ! The real 

 history of the green-bearded oysters is now quite certain, 

 and any one interested in the matter should look at the 

 coloured pictures of the beautiful little blue-coloured 

 Navicula ostrearia the diatom on which this oyster 

 feeds, published by me in the Quarterly Journal of Micro- 

 scopical Science in 1885. 



