THE LIFE-HISTORY AND HABITS OF THE PEARL OYSTER. 135 



valve, its first movement before making a byssal re-attachment is to protrude the foot 

 to the utmost extent, and give a violent contraction so as to cant the shell over on to 

 the right side. When lying undisturbed the pearl oyster separates the ventral edges of 

 the shell to the extent of about g- inch. This aperture is, however, reduced to a mere 

 slit by the inwardly directed edges of the pallial lobes from either side which nearly 

 meet, and interlock by means of the marginal digitate tentacles. 



Through this sieve-like slit, guarded by these sensitive branched processes, an 

 indraught of water is carried by the constant lashing of the cilia covering the gill 

 surfaces. Any large particle unsuitable for food, or any intruding animal touching the 

 pallial tentacles, causes the valves to close with a snap. Otherwise a constant stream 

 of small particles, diatoms, spores, protozoa, and other microscopic organisms, is carried 

 in with the water flowing towards the branchiae, which function as very fine 

 strainers, able to sift out and arrest every particle from the incoming current. The 

 food particles so arrested are carried by the cilia along the furrows to the crest of the 

 branchial lamella. There the band of specialised cilia forms a path from end to end, 

 along which the particles pass forwards at a rapid rate to the anterior branchial apex, 

 where they are carried into the palpar gutter of that side. In all, four ciliated 

 pathways lead along the branchial crests to the palpar gutters, two on each side. 



Occasionally, too, particles find their way into one of the six ciliated paths at the 

 base of the branchial lamellae. In this case they may either be propelled forward along 

 this basal pathway, or, as usually happens, be drawn away, after travelling a short 

 distance, by the ciliary current of the branchial furrows, and so be transferred to the 

 crest and follow the normal course. 



Observations show that the palps can exercise a power of rejection. Thus if there 

 be sediment in the water, the particles of mud after being sifted out by the gills and 

 passed to the palps are retained by the latter until enough accumulate to form a tiny 

 pellet. This is rotated slowly just within the truncated ends of the palps for a tune, 

 and is then suddenly ejected by a slight movement of the palps on to the surface of 

 the broad ciliated band that runs outwards to the pallial edge from the base of the 

 palps. Along this the rejected pellet is carried, round the inside of the pallial edge, 

 till, arriving opposite the posterior tips of the branchiae, it is thrust out. The 

 ciliated path there terminates at the distal end of the lip-like twist of the pallial 

 margin on the ventral border of the exhalent orifice. As the pellet leaves the pallial 

 edge it comes under the influence of the excurrent stream from the gills, and thus is 

 carried some little distance away from the oyster. 



On the other hand, nutritious particles received on the palpar surfaces from the 

 branchial ciliated paths are passed on at once to the mouth. 



V. Shell-Growth and Repair. 



Such subjects as rate of normal shell-growth, power and extent of shell-reparation, 

 and cases of arrest of growth, are all closely related, and may be considered together. 



