INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 931 



Green Colour of Oysters.* — lu 1877,+ mention was made of the 

 fact that the green cohmv observed in oysters, in certain localities, is 

 caused by a variety of Navicula, to whicli the name N. oslrearia has 

 been given. Further particulars of experiments made by M. Puysegur, 

 at Sissablc, are not without interest.| 



" The green slime was collected by lightly scraping the margin of 

 one of the ' clears ' with a sjioon, and was put in flasks, shaken for a 

 moment, and then allowed to settle, so as to get rid of the mud, some 

 admixture of which is inevitable. The coloured fluid, containing 

 little or nothing besides diatoms, was then poured ofl' into other flasks. 

 Care and some little dexterity are requisite, as if there is too much 

 silt, or too large a quantity of water, which is generally the case 

 when the task is intrusted to a subordinate, it is sometimes next to 

 impossible to concentrate the fluid enough to show the results with 

 the desired plainness. 



Returning home, we poured the fluid into soup-plates set on a 

 table before a window. The diatoms speedily settled on the sides 

 and bottoms of the plates, coating them with a green slime, the thick- 

 ness and tint of which varied with the jiroportiou of diatoms present. 

 In each plate, according to its size, we put three to six i)erfcctly 

 white oysters, which had never been in the ' clears,' and the shells of 

 which had jjreviously been washed and brushed clean. In similar 

 l)lates like numbers of the same oysters were laid in ordinary sea- 

 water. Twenty-six hours after the ccmimencement of the experiment 

 the oysters in tlic water charged with diatoms had all acquired a 

 marked grecnisli hue ; the other oysters remained unaltered. The 

 experiment was repeated many times with identically the same results. 

 The green colour in the oysters was found to be more decided in 

 l)roportion as tlie water was more highly charged with diatoms. 

 In the course of the experiments the shell of one of the oysters was 

 l)erforated, so as to lay bare tlie mantle. After the oyster had turned 

 green, it was laid in ordinary sea-water for a few days, when the 

 greenness disapi)eared altogether. It reai)peared when the oyster was 

 replaced in fresh water containing Navicula ostrearia. 



In the course of tlie exi)eriments it was observed that by the open- 

 ing and closing of their valves the oysters induced currents in the 

 water, by means of which they drew towards them, and surrounded 

 themselves with, the particles of matter suspended therein. The 

 existence and direction of these currents were shown by the dis- 

 ajjjtearance of the slime and the consequent laying bare of the sides 

 and bottoms of the i)lates, the diatoms remaining only at jjoints out 

 of reacli of the currents. Directed towards the biitcal ajierture by 

 the cilia with whicli the branehiie are provided, the Navicahv. enter 

 the stomach of the molluse, and there part with their nutritive con- 

 stituents. The yellow chlorophyll is digested and decomposed ; the 

 soluble colouring mutter passes direct into the blood, to which it 

 iiiij)iu-ts its colour. Thus it happens that the most vesicular portions 

 of the structure, us the bruuchiic, arc the most highly coloured. 



Examination of the digestive tubes of the oystcx's cxporimeuted 



• ' Nature,' xxii. (ISSd) )). ."ilil. t H'l''-. xvi. (1877) p. :{;>7. 



\ ' liivuf iiiuiilime tt (.uldiiiiilc,' l-Mirunry ISSO. 



