GREEN OYSTERS. 17 



they are epithelial cells (" Beckerzellen ") ; we also showed 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.f 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 apparently 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 Cyanophycea;, such as Microcoleus 

 chthonoplastes, Lyngbya majuscula, and L. semiplena. 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 February, 1897, by Dr. J. L. C. Pompe 

 van Meerdervoort, who complained of a diffused greenness which appeared in October 



"Contribution \ I'Etude de la Phagocytose, 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 Mersea. 



