OYSTERS AND OYSTER EISHERIES OE QUEENSLAND. 265 



adhere to the cement by the entire surface of the attached shell. After attaining about one-half 

 of an inch in diameter, the free edges of the shells commence to grow outwards ; and this direction 

 of their growth is continued until, at an age of about six months, they project an inch and a-half or 

 two inches from the collector. At this stage, the young oysters may be easily detached with or 

 without the cement, and be laid on the banks as ordinary "cultivation." The collectors may then 

 be re-cemented and re-laid, for the catchment of a second crop. Cemented slates have also been 

 found to prove very efficacious spat collectors, and they most nearly approach the French tiles. 

 Slates, procured as an ordinary market article, are too expensive for general employment. It 

 sometimes happens, however (as occurred with the supply experimented with), that condemned 

 lots may be obtained at but a little over the cost of carriage, direct from the quarries. In 

 utilising slates for this purpose, it has been found most convenient for manipulation to use them 

 in combined series, wiring sets of half-a-dozen or so to each side of a ridge-shaped frame con- 

 structed of light saplings cut to four or five feet lengths. 



Characteristic illustrations, from photographs, of the Australian " split-paling " oyster-spat 

 collectors are introduced at page 278, as a tail-piece to the present chapter. One figure depicts 

 the collector in its newly-constructed form, ready for introduction into the water, and the 

 second one represents a collector, which, after remaining in the water for three months, 

 is completely covered with oyster brood. 



DESTRUCTIVE AGENCIES AND DISEASES. 



Among the enemies from whose attacks the Queensland oyster-grower sufTers serious loss of 

 stock, the small boring whelk, Urosalpiiix pavicv of Crosse, must be awarded a prominent position. 

 The destructive influences exerted by this mollusc are parallel to those wrought by Miire.x 

 farciifiiius and Nassa rcficiilafa on the European oyster-beds. The shell ol the Queensland borer, 

 while much like that of the last-named European species, is more slender, and has a distinct 

 violet-coloured lining to the mouth aperture. The young oyster, from its earliest attached condition 

 up to about one-half of its adult growth, is much subject to the ravages of this foe. Not unfre- 

 quently, it is so abundant and predaceous that almost the entire brood-stock on a bank, or from 

 dredgings, investigated, was found to be destroyed by this borer. Mature oysters, even, are not 

 exempt from the attacks of this enemy, one of the shells in the typical cluster illustrated by 

 Chromo plate XIV., Fig. 8, exhibiting at "A " the symmetrically circular hole by which the Urosal- 

 pinx has gained access to its prey. The drilling operation is performed by these boring molluscs 

 with the aid of a toothed ribbon, technically known as the "radula," which is enclosed within, but 

 can be partly protruded from, the buccal cavity. It has been ascertained by direct experiment in 

 connection with the European species, Miire.x taraitiniis, that the mollusc takes about half an hour to 

 pierce the shell of a young oyster one month old, and eight hours to perforate a matured one of three 

 years' growth. The only practical remedy for the destruction of this pest, so far employed, is hand 



M M 



