Gtttli) }[i>rine Lnhoralorifj St. Andrews. S07 



mud ; and a coii.siderable number of years ago the Whitstable 

 Ovster Com))aiiy of the Tiianics were greatly exercised on 

 tliis subject, for they were of opinion that the deposition of 

 mud from the dredgers used in the Thames was the cause of 

 the attacks which rendered many of their beautiful oysters 

 unsightly. The Pohjdora (P. cih'ata) which is found in 

 tlu'se is much smaller, as a rule, than that {P. hophira) in the 

 Australian oysters; but if the authorities of New Houtli 

 Wales saw the havoc made of the Whitstable shells, they 

 would derive some satisfaction from the fact tiiat things 

 might be worse than they are in Australian waters. Yet 

 many of the Whitstable oysters, though honeycombed ex- 

 ternally by the worm-tunnels, are perfectly sound within, and 

 fit to appear in the most fastidious market. Probably much 

 dej)ends on the vigour of the oyster, as in some cases, instead 

 of the Australian " blisters," the large Whitstable oysters 

 pour out a dense mass of pure white calcareous secretion 

 nearly a quarter of an inch in thickness, thinning off at the 

 edges over an area of 1^ in. square, and thus keep the lively 

 annelids at a distance from the mantle. Of the two com- 

 mensals, the annelid — British or Australian — is preferable to 

 the sponge, yet it is doubtful if the losses through them are 

 so serious as to endanger the success of oyster- culture either 

 at home or in Australia. 



Mr. Whitelegge proposes to kill the boring-worms by 

 immersing the oysters in fresh water for some days, or by 

 ])lacii)g them in the air (under shelter from the sun) for ten 

 days (ir longer. It is quite possible that the annelids would 

 thus be killed, and, it may be, some of the oysters. No 

 sooner, however, would the oysters be replaced on any 

 ordinary bed within reach of sea-water tlian they would be 

 liable to inroads both of the annelids and the sponge, since 

 the larval and postlarval forms of both abound, the former 

 especially for many months. Besides, unless in special 

 enclosed parks, any oysters left on the beds, or the occurrence 

 of old shells, limestone bouldtrs, suitable shale, or even sand- 

 stone in the neighbourhood, would re-people the entire area 

 within a brief space. 



The perforations of Poli/dora in oysters need not be taken 

 too seriously, since even the best oysters are thus invaded, 

 and yet their quality is undisputed. The condition is so 

 universal that the greate.-st ditticulty would be encountered in 

 stamping it out, and it is doubtful if the result would com- 

 pensate the labour and expense, even were it practicable. 

 Besides, as already mentioned, the boring-sponge is a more 

 dangerous enemy to the oyster-shell, since it so generally 

 perforates the inner or nacreous layer. 



