THE LIFE-HISTORY AND HABITS OF THE PEARL OYSTER. 129 



After a .size of about - 175 is reached, prismatic shell substance begins to be 

 deposited marginally, as seen in fig. 42, II., III., IV. The embryonic shell persists, but 

 the shape of the valve changes to one which is recognisable as the adult type. The 

 umbones become less prominent, and, as a result of the asymmetrical marginal growth, 

 become more anterior in position. Even at this early period the byssal sinus (by.*., 

 fig. 42) is recognisable, and a verv delicate byssus can be seen anchoring the little 

 Mollusc. If the spat be attached, as the young stages, in our experience, generally 

 are, to floating or for a time rooted Algae, they seek the under, shady, side of the 

 branches, and sooner or later drop off and sink to the bottom, being dislodged either 

 by shaking or by the disintegration of the floating weed The further history will 

 then depend upon whether they fall upon ground where there is suitable " culch " for 

 attachment, or upon sandv areas where they probably get buried, scoured out, or 

 rasped to pieces In - the constant movements of the sand grains. The enormous 

 number of minute shells of the fry or spat of pearl oysters which we found in some 

 sandy deposits indicates the extent of the destruction which is going on, and which 

 might, in part at least, be prevented by artificially increasing the area covered bv 

 -culch." 



We have also evidence that the ground may vary from year to year in the covering 

 of fine Algse which it affords for the attachment of the young spat. This is shown bv 

 the following observations : 



On the south end of the Cheval Paar, not far from the Shoal buoy, on February 26th, 

 1902, we hauled up great masses of mostly green delicate Algre thickly covered with 

 oyster spat. The Algse proved to be Hypnea musciformis, along with a Chidophora, 

 a Ceramium, and some Cluetomorplia and Polysip>honia, mostly infested with 

 Cyanophyceai and various microscopic animals. Much the same assemblage of plants, 

 but of a redder colour, was found on the Modragam paars on March 10th, and we 

 also obtained them in smaller amount elsewhere on the pearl banks. In all cases 

 these weeds supported vast quantities of spat. Later on in the season we found 

 masses of the weed detached and floating on the surface. This season, however (1903), 

 Mr. Hoknell finds, after a careful examination of the same grounds in a diving suit, 

 that the weed is entirely absent, and that the sand in these localities is bare and 

 shows no spat. This marked difference in the condition of the bottom on two 

 consecutive years throws an interesting light on the factors which may determine on 

 occasions the productivity of a bed, and lends support to our contention that putting 

 " culch " on the bottom at selected spots would lead to an increase in the amount of 

 spat deposited. 



III. The Byssus and Locomotion. 



Although the formation of the byssus threads comes more appropriately in the 

 section on the Structure of the Pearl Ovster, still the connection of the byssus with 



s 



