194 
THE NATIONAL GEOGRAPHIC MAGAZINE 
some brief mention thereof be made, 
because this subject has received special 
elucidation in Ceylon. 
During the first 1,500 years of the 
present era, and doubtless for many pre- 
ceding centuries, every theory of pearl 
formation had as its essential feature the 
idea that every pearl was originally a 
drop of dew or rain — possibly a tear — 
that gained entrance into the shell of an 
oyster in one of various ways. Pliny 
the Younger, in his celebrated Natural 
History, gives a detailed description of 
this process, and similar accounts appear 
in the writings of philosophers, travelers, 
poets, and others in ancient, medieval, 
and even early modern times. 
It is a noteworthy fact that at the 
present time the Arab, Persian, and 
Indian divers quite generally believe that 
at certain seasons the pearl oysters come 
to the surface in the morning, open their 
shells, and suck in or imbibe in some way 
a dewdrop or raindrop, which, suffused 
with sunlight, is slowly transformed into 
a lustrous pearl. The American consul 
at Aden recently reported that the scar- 
city of pearls in the Red Sea was as- 
cribed by the Arabs to the fact that little 
rain had fallen for several years. 
We now know that almost any kind of 
foreign body — whether a grain of sand, 
a bit of mud or shell, a piece of seaweed, 
or a small animal — may by its irritation 
cause the mollusk to cover it with nacre 
and make it the nucleus of a pearl. The 
pearly matter is slowly deposited in defi- 
nite layers, and the growth of the pearl 
continues indefinitely. 
I But if the annual supply of pearls de- 
pended on the foreign bodies accidentally 
gaining entrance into the cavity of the 
pearl oyster, there would be no great 
pearl fisheries, and pearls would not be 
the highly prized, costly gems they are. 
It has now been pretty definitely es- 
tablished that the great bulk of the an- 
nual pearl crop of the world — probably 
90 per cent of it — represents animal 
parasites which normally pass a part of 
their life-cycle within the pearl oysters, 
and during that period, becoming en- 
capsuled in the tissues of the mollusk, 
are in time covered with a nacreous coat, 
owing to the irritation they impart to the 
oyster. It was not until the middle of 
the 19th century, however, that the para- 
sitic origin of pearls was proposed and 
established, and some of the earliest re- 
search was addressed to the Ceylon 
pearl oyster. But it was only during the 
present century that the true role of the 
parasite and its life history were satis- 
factorily cleared up. 
It is now known that the minute 
spherical larvae of various marine worms, 
but particularly of cestodes, enter the 
pearl oysters and become more or less 
embedded in the soft tissues, as many as 
40 of these larval worms having been 
found in one Ceylon pearl oyster. As a 
result of the irritation caused by a larva, 
the oyster forms a protecting epithelial 
sac about the intruder, and then, if the 
latter dies, its mass is gradually con- 
verted into carbonate of lime, pearly 
nacre is secreted by the contiguous epi- 
thelium, and the growth of the pearly 
mass proceeds with the growth of the 
shell which is formed in the same way. 
Reference has been made to the life- 
cycle of the parasite. If the larvse do 
not die, the hosts may be eaten by fishes 
and the larvae will not find lodgment 
therein and undergo a certain develop- 
ment. Among the fishes that largely 
prey on the pearl oysters are the tough- 
skinned, strong - jawed trigger - fishes. 
These in turn are eaten by large rays 
that are common on the pearl-oyster 
grounds, and in the rays the worms 
reach their full development and pro- 
duce young (larvae) that are cast into 
the water and find lodgment in the oys- 
ters. 
We are thus prepared to accept the 
well-known saying of a celebrated 
French investigator, that "the most beau- 
tiful pearl is in reality only the brilliant 
sarcophagus of a worm." 
