466 
NATURE 
[Marcu 17, 1904 
tenantless. | Occasional breaks of five years or less 
account for the remainder of these years of famine. 
But this apparently inexplicable state of affairs is by 
no means of modern date; for centuries these fat and 
lean seasons have been the despair of those interested 
ia the collection of these gems. 
All kinds of theories, or rather speculations, have 
been promulgated, but the mystery remained, until 
now, as impe snetrable as ever. U ndoubtedly, then, the 
fisheries called 
of the Government, not 
immense importance and value of these 
for some effort on the part 
only to attempt a solution, but also to find a remedy 
for these years of failure. To this end the Colonial 
Office. acting on the advice of Prof. E. Ray Lankester, 
invited Prof. Herdman to examine the records on this 
subject and to report to them. As a result of this re- 
port Prof. Herdman was induced by the Government 
to make a personal inspection of the pearl banks. 
Taking with him Mr. Hornell as_ his scientific 
assistant, he accordingly set out for Ceylon, and in- 
stituted a thorough examination of the whole question, 
the results of which are set out, in part, 
in the volume before us. Without doubt 
the task that lay before these investiga- 
tors was no light one, and it is equally 
certain that it has been admirably fulfilled. 
As the pages of this volume show, the ex- 
pedition has not only been abundantly 
fruitful in scientific results. but it has 
achieved the purpose for which it was dis- 
patched. 
A complete survey of the whole sea 
bottom of 
been made, 
f the pearl fisheries area has now 
partly by sounding and dredg- 
ing and partly by the aid of divers. In 
some cases even, Mr. Hornell himself de- 
scended in a European diving dress. By 
this saree’ a thorough knowledge has 
been gained, not only of the nature of the 
ground best suited for the growth of the 
pearl oyster, but also of the dangers by 
which this animal is beset. 
Flourishing beds may be depleted by 
the ravages of boring sponges, boring 
molluscs, starfishes, internal parasites, 
and fishes, though the destruction wrought 
by these is generally slight compared with 
the wholesale destruction caused by shift- 
ing sands due to currents, or churned up 
by the south-west monsoon. By way of 
illustrating the vast scale of catastrophes 
of this kind, an instance—one of several— 
may be selected here. One bed examined 
in March, extending over an area of sixteen square 
miles, was covered with enormous quantities of young 
“so closely packed that the banlk must have 
held not less than about a hundred thousand million.”’ 
Early in 
Fic. 
oysters 
November of the same year this spot was 
revisited, when this vast host was found to have 
vanished, having been buried in the sand or swept 
down the deep declivity outside the bed. 
The loss which results from overcrowding is, on 
scme beds, almost as. wholesale. If, however, Prof. 
Herdman’s suggestions are carried out, this enormous 
waste will in future be prevented by the simple ex- 
pedient of transplanting to sheltered spots affording 
suitable conditions for growth and infection. Nature 
has often to be assisted in the preparation of these 
spots by the process known as “ culching,” that is, 
scattering the floor of the bed with rock, loose coral, 
and so on, to afford the necessary anchorage for the 
byssus of the young oyster. 
What havoc may be caused by starfishes can be 
gathered from the fact that a bank examined in 
NO. 1794, VOL. 69] 
1.—Pentaceros lincki, DeBl., 
l 
March, 1902, lodged a crop of oysters estimated at 5? 
millions ; by "March, 1903, they had nearly gone! 
Over-fishing is another source of danger, though in 
future, if Prof. Herdman’s plan of transplanting is 
judiciously carried out, this need not be teared. In 
many places, it has been discovered, fishing may be 
carried on by dredging instead of by diving, though 
there are many places where the nature of the bottom 
will still compel the services of the native diver. 
On the question of the formation of pearls this book 
contains much of great interest, and yet fuller details 
are promised in the next volume. Only in extremely 
rare cases did these investigators find that the nucleus 
of a pearl is formed by a grain of sand. Boring 
sponges and burrowing worms cause the formation of 
pearls or pearly excrescences on the inner surface of 
the shell by the irritation which they set up. Pearls 
of a peculiar kind are found in the muscular tissues, 
usually the levators of the foot. These also have no 
organic nuclei, but seem to start as minute calcareous 
concretions, and may be extraordinarily abundant. 
lying on a large pearl oyster, half natural size. 
From a photograph by J. Hornell. 
Thus, at the insertion of one of the levator muscles 
23 small pearls were detected with the nalsed eye, 
whilst under the microscope 170 more ie) spherules 
were found. But the best ‘‘ orient’ “cyst ”? pearls 
are those which occur in the Fantle * ‘*or in the thick 
white lateral part over the stomach and liver, or even, 
secondarily, free in a cavity of the body.’’ Caused by 
the secretion of concentric layers of nacre around the 
dead body of a parasite—generally that of a platy- 
helminthean larva—these pearls attain their greatest 
size in oysters of from three and a half to five years 
of age. 
This parasite has an interesting history. _Com- 
mencing life as a free-swimming embryo, it, in favour- 
able circumstances, finds an entrance between the open 
valves of the oyster shell, or is drawn in by inhalent 
currents. The entry once gained, the next step is to 
bore into the tissues of the host, and here it undergoes 
the early stages of its growth. If the fates are pro- 
pitious the host is eaten, and the developing worm 
escapes uninjured from the body of its first into that 
