114 FALL OF THE SPAT POISONOUS OYSTERS chap. 



piece in five or six months, and in a year to 1 inch in 

 diameter. Eoughly speaking, the best guide to an oyster's age 

 is its size ; it is as many years old as it measures inches across. 



The oyster is at its prime at the age of five ; its natural life 

 is supposed to be about ten years. The rings, or ' shoots ' on a 

 shell are not — as is frequently supposed — marks of annual 

 growth ; cases have been noticed where as many as three 

 ' shoots ' were made during the year. 



An oyster is furnished, on the protruding edges of the 

 mantle, with pigmented spots which may be termed ' visual 

 organs,' though they hardly rise to the capacities and organisation 

 of real ' eyes.' But there is no doubt that they are sufficiently 

 sensitive to the action of light to enable the oyster to apprehend 

 the approach of danger, and close his doors accordingly. ' How 

 sensitive,' notes Mr. W. Anderson Smith,^ ' the creatures are to the 

 light above them ; the shadow of the iron as it passes overhead 

 is instantaneously noted, and snap ! the lips are firmly closed.' 



The geographical distribution of Ostrea edulis extends from 

 Triinen, in Norway, close to the Arctic circle, to Gibraltar and 

 certain parts of the Mediterranean, Holland, and IST. Germany to 

 Heligoland, and the western shores of Sleswick and Jutland. It 

 occurs in Iceland, but does not enter the Baltic, where attempts 

 to colonise it have always failed. Some authorities regard the 

 Mediterranean form as a distinct species. 



The literature of oyster -cookery may be passed over in 

 silence. The cm-ious may care to refer to M. S. Lovell's Edible 

 British Mollusks, where no less than thirty-nine different ways 

 of dressing oysters are enumerated. It may, however, be worth 

 while to add a word on the subject of poisonous oysters. Cases 

 have been known where a particular batch of oysters has, 

 for some reason, been fatal to those who have partaken of them. 

 It is possible that this may have been due, in certain instances, 

 to the presence of a superabundance of copper in the oysters, 

 and there is no doubt that the symptoms detailed have often 

 closely resembled those of copper poisoning. Cases of poisoning 

 have occurred at Eochefort through the importation of 'green 

 oysters' from Falmouth. It would no doubt be dangerous ever 

 to eat oysters which had grown on the copper bottom of a ship. 

 But copper is present, in more or less minute quantities, in very 



^ Bendcrloch, p. 136. 



