GREEN OYSTERS. 
17 
they are epithelial cells (" Beckerzellen "j ; we also shuwed that certain other eosino- 
philous cells in the epithelium were clearly wandering leucocytes, and of the same 
nature as the wandering cells met with in the connective tissue. 
C. de Bruyne,* in his recent extensive work on Phagocytosis in Lamellibranchs, 
shows that in these molluscs there is a considerable exodus of leucocytes on the surface 
of all epithelia, both mucous and tegumentary. He considers that the diapedesis of 
leucocytes is always very important in the branchial epithelium, and becomes increased 
in the presence of any irritation. Our experiments described further on (p. 20) show 
that this is certainly the case in the oyster. 
We regard the wandering cells with green granules, whether eosinophilous or not, 
which we find so plentifully in the epithelium of the gills and mantle {see Fig. 3), as 
leucocytes which are on their way to the surface, and as being of the same nature as 
the wandering cells we meet with in the connective tissue. 
2. Roach River Green Oysters.— Certain parts of the estuaries of the Roach and 
the Crouch, in Essex, have long been known to produce green oysters ; but these are 
usually regarded with so much suspicion in this country that, although healthy and 
delicately flavoured, they have to be sent abroad to find a market. We give a figure 
(PI. IV., Fig. 9) of one of these oysters,t showing the rather pale blue-green colour of 
the gill. We find nothing abnormal or unhealthy in these oysters, and no unusual 
amount of copper or other metal in the green parts. The structure of the branchiae 
and the cause of their colour are ap[)arently the same as in the case of the Marennes 
oyster. 
On the Essex coast, just as in the case of the French " claires," the Oyster- 
growers notice that immediately before the oysters become green, in autumn, a large 
amount of fine green weed or " moss " makes its appearance in the estuaries in question. 
We are indebted to Mr. E. Newman, manager of the Colne Oyster Company, for 
obtaining samples of this so-called " moss " for us to examine. We are now able to 
state that this green layer which appears as a coating on the mud ("London clay"), 
is a matted mass of minute Algae, almost wholly Cyanophyceae, such as Microcoleus 
chthonoplastes, Lyngbya inajusmla, and L. seviiplena. This is very similar, then, to the 
" verdure " of the French " claires " we have examined, except that in the latter several 
species of Cladophora were also abundant. {See Herdman " On Oyster Culture on the 
West Coast of France," Trans. Biol. Soc. Liverpool, vol. VIII., p. 113). In both cases 
many Diatoms are present, and these, along with the blue-green Cyanophycea;, must 
form an important part of the oyster's food. 
3. Green Dutch Oysters.— A large number of Dutch oysters from Nieuport, in 
Belgium, were sent over to us in January and P"ebruary, 1897, by Dr. J. L. C. Pompe 
van Meerdervoort, who complained of a diffused greenness which appeared in October 
* Contribution a I'Elude de la I'hagocytose, Arch, de Biol. t. XIV., 1895 ; also t. XV., p. 270, 1898. 
tFrom a batch kindly procured for us by Messrs. Cooke Bros., Oyster Merchants, West Mer:,ea. 
