THE LIFE-HISTORY AND HABITS <>F THE PEAK! OYSTEK. 1.45 



suffice to show the rapidity and thoroughness with which the pearl oyster makes 

 good injury to its shell, and to effectually dispose of any objections that might he 

 raised to extensive transplantation on the score that the oysters might be damaged 

 and would not survive the operation. 



Observations on the damaged oysters kept in our aquarium tanks at Galle throw 

 light upon the methods of repair. Damage may either be marginal or superficial. If 

 the latter, the cause is usually a blow fracturing one of the valves through both 

 nacreous and prismatic layers and depressing the shell on the distal side of the 

 break. In a specimen purposely so injured, the mantle-lobe separated from the 

 proximal part of the shell for some \ inch, leaving a wedge-shaped recess under the 

 shell-margin at the fracture. In this new position the external pallial epithelium 

 secreted a fresh deposit of nacre, thus perpetuating permanently the crevice under the 

 proximal fractured edge. 



Cases where the mantle is extensively pierced or lacerated by the injury may be 

 fatal, not apparently because the animal is unable to make good the damage, but 

 rather because of the difficulty of keeping out small carnivorous animals such as 

 worms and molluscs. In several cases, however, we have seen complete recovery from 

 extensive laceration, the margins of the wound gradually approximating and finally 

 uniting. While the temporary aperture exists, water may be drawn in through it, 

 and the tip of the foot is sometimes protruded through the hole in the mantle and 

 moved round the rough edge of the broken shell, with the result that loose fragments 

 and dirt particles that accumulate between the mantle and the nacre, and which 

 probably cause irritation, are removed. No repair-nacre is secreted until the damage 

 to the mantle is made good. 



When a fragment of the shell-margin is broken away, the mantle within is retracted 

 in proportion to the extent of the damage, not only at the place of injury but for 

 some distance on either side. Consequently, when the pallial edge begins to form 

 a new shell-margin, it starts in the uninjured part from a point about -J- to yo r mcn 

 behind (internal to) the former edge. Along the line of fracture the pallial edge 

 is advanced much closer, or quite up to the margin, beyond which the pallial tentacles 

 can be seen projecting. This method of starting the new growth behind the existing 

 edge in the case of injuries is also the normal method of adding to the margin 

 f the valves in growth. Under normal conditions growth is discontinuous, periods 

 of activity alternating with what appear to he periods of rest. Each fresh layer 

 begins from a line about yV inch inside the edge; which process results in the 

 formation of a number of successive layers of shell with projecting margins outside. 

 the older overlapping the younger in imbricate fashion. Young rapidly growing 

 pearl oysters show this imbrication most markedly, especially during the first two 

 years. As they get older, attrition and the ravages of attacking and incrusting 

 animals tend to remove the thin projecting margins and .their delicate processes, and 

 new ones are less frequently formed as the shell ceases to grow in extent hut becomes 



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