74 PROFESSOR RAY LANKESTER. 
breadth. From 500,000 to 600,000 oysters can be placed in 
each of these tanks. Tanks of this character are used by 
oyster merchants at Marennes, Oleron, Courseulles, Caen, 
Havre, Dieppe, Tréport, &c. At certain seasons of the year, 
particularly from April to June and again in September, the 
water in these reservoirs acquires a dark bluish-green tint. 
This is due to the growth in the tank of a particular species of 
Diatom—the Navicula ostrearia—which was observed with 
the microscope by M. Gaillon (sixty-five years ago!) and was 
called by him Vibrio ostrearius. M. Gaillon describes the 
enormous abundance of these Diatoms and their characteristic 
movements; he also notes their colour. 
No figure of the Navicula ostrearia in its living con- 
dition, showing its beautiful combination of blue and golden 
colours, has, I believe, ever yet been published, and accordingly 
in Plate VII, figs. 1 to 9, I have given a series of coloured 
drawings, made from specimens which I received in the living 
state in London from the Director of the Botanical Laboratory 
of Le Croisic (Bretagne), whose kindness in supplying me with 
this material I desire to record. I am also indebted to the 
distinguished botanist, M. Bornet, for placing me in communi- 
cation with this gentleman, and to my friend Dr. Vignal, of 
the Laboratory of General Anatomy in the College de France, 
Paris, for very kindly sending me on two occasions a hamper- 
full of “‘ huitres de Marennes” from Paris. 
M. Gaillon relates, in his memoir of 1820, that the oyster 
merchants carefully place the colourless Oysters dredged from 
various oyster-beds in the tanks where the Navicula 
ostrearia has multiplied to such an extent as to colour the 
tanks green. After a few weeks the Oysters, previously colour- 
less, are found to have assumed a bluish-green colour in the 
gills and labial tentacles. If the Oyster is removed from the 
tanks containing the Navicula, or when the Navicula 
growth dies down, the Oysters gradually lose their colour, so 
that in the course of a month Oysters which were deeply 
coloured will have only the faintest trace of a green tint. This 
disappearance of the green colour when the Oyster is kept in 
