1883.] on Oysters and the Oyster Question. 353 



mischief done in tliis way by a burrowing annelid {Leucodore^ was 

 recently brought to my notice by Sir Henry Thompson. The 

 Leucodore drives burrows into the shell and lives in them without any 

 evil intent towards the ovster. But the burrows fill with fine mud, 

 and this, spreading into the vacuities of the shell, gives rise to inky 

 patches, which look unpleasant when the oyster is opened and damage 

 its commercial value, though, as I can testify, the flavour of the oyster 

 is nowise impaired. 



The larval oysters are extremely sensitive to cold, and any sudden 

 fall in the temperature of the air during the swarming time is fatal to 

 them. Even the adult oyster is readily killed by sudden frosts, if the 

 water is sufficiently shallow to allow the change of temperature to 

 penetrate. Great heat is equally pernicious. At Arcachon, immense 

 numbers of oysters were killed by the hot summer of 1870. 



To this long list of influences against which every oyster has to 

 struggle successfully, if it is to attain maturity, larger knowledge will 

 doubtless add many others. But these are enough to enable us to 

 understand why it is that the increase of a given stock of oysters may 

 be, and usually is, very slight, notwithstanding the prodigious fertility 

 of the individual oyster, A very large proportion of the oysters in a 

 bed, under ordinary circumstances, breed during the season ; and, as 

 each adult female oyster, on an average, gives rise to a million eggs, 

 one would expect a prodigious increase, even if nine-tenths of the 

 young were destroyed. But from the small proportion of half-grown 

 to full-grown oysters (40-50 per cent.) it is clear that the real addition 

 to the oyster population, in most years, is very small. It is probable, 

 in fact, ihat unless the conditions are unusually favourable, not more 

 than two or three out of every million of the fry of the oyster ever 

 reach maturity. 



It is obvious that the conditions of existence of the oyster are of 

 an extremely complicated character, and that the population of an 

 oyster bed, under natural conditions, must be subject to great fluctua- 

 tions. A few good spatting years, accompanied by a falling off in 

 the number of starfishes and dogwhelks, may increase it marvellously, 

 while the contrary conditions may as strikingly reduce it. 



Man interferes with this state of things in two ways. On the one 

 hand he is one of the most efficient of destroyers, and on the other, he is 

 the only conservator of the moUusks, albeit his conservation is with a 

 view to ultimate destruction. Let us consider him first under the 

 aspect of destroyer. In some places, oysters are taken at low tide by 

 the hand ; but usually they are captui-ed by means of the dredge, 

 which is essentially a bag, the sides of the mouth of which are 

 fashioned into scrapers. The di'edge is di'awn slowly over the oyster 

 bed for a certain time, and the oysters, with multitudes of other 

 animals, stones, and the' like, are scraped into the bag. This is then 

 hauled up, and the contents emptied on to the deck ; the oysters are 

 picked out and the refuse returned to the sea. 

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