April i, 1904.] THE TROPICAL AGRICULTURIST. 
705 
ever, in the neighbourhood of paars otten bears 
oysters in clumps adhering to a fragment of dead 
coral, an old Mollu'^c n s-hell, or more frequently a 
Nullipore nodule ( LitJiotliumnion). Such pieces of 
natural ''culch" are of enormous importance to 
the prosperity of the fisheries, and the area covered 
by them mifiht be largely increased by artificial 
" culching." The TEMrRRATURE of tlie sea water 
in which the pearl oysters live in the Gulf of 
Mannar is high. In our expei lence it ranges from 
about 77° F. in January to close on 90° F. in Apiil. 
Probably the normal range during the greater 
part of the year is from 82° to 86° F. The SPKCIKIC 
GRAVITY we found to be fairly constant at. 1'023 
on the pearl banks ; at Galleit was slightly lower 
averaging 1"022; at Trinconialee in the inner bay 
and specially in Tampalakam, it was distinctly 
lower (1*015 to 1"019). At exceptional spots and 
seasons in the Gulf of Mannar we found the 
specific gravity lower than the normal. Off 
Chilaw in November, 1902, it was a little above 
1 '019, and on the Muttuvaratu paar in the same 
month it varied about 1*020. N.o doubt on occasions 
of great floods on the laud it may be lower still 
on those paars that are near the mouths of the 
rivers. There is no reason to think (as has beet) sug- 
gested) that some admixture of fresh water is necess- 
ary for the prosperity of the oyster or for pearl for- 
mation. On the contrary exceptional floods are pro- 
bably harmful to any paars they may reach. 
There is a general drift of the water over the banks 
from south to north from April to September and 
from north to south during the height of the 
north-east monsoon, with intermediate periods of 
calms and variable winds from February to April, 
and usually again in Movember. But we are still 
in want of more definite information (such as can 
only be obtained by some years of observation 
and experiment with "drifters") in regard to 
nsual surface drift during the periods of variable 
winds between the monsoons before we can be 
certain of the source of "spat" supply to 
particular banks, or of the destiny of larvre 
produced from our adult oysters. "Drift-bottle" 
experiments, such as have been recently made for 
fi^iheries purposes in several European seas, should 
be instituted in the Gulf of Mannar, It is only 
after such work has been carried on systematically 
for two or three years at least that it will be 
possible to determine the 
COURSE TAKEN BY THE LARVAL PEARL OYSTERS 
between the time of hatching and the deposit 
of spat, and again between the attachment to 
floating Algrc and the appearance of young 
oysters on a paar. These are details which it was 
impossible for us to settle in the time at our dis- 
posal in 1902, but which will naturally in the 
future form an important part of the work of a 
Marine Biologist resident in Ceylon. The 
FAUNA AND FLORA OF THE GULF OF MANXAR, 
comprising the whole assemblage of plants and 
other animals large and small, which surround the 
Eearl oyster, has a profound effect upon the well- 
eing of the oyster and of the pearl fisheries. We 
took every opportunity of investigating this fauna 
and flora and the results are being reported upon 
In detail. It will suffice to point out here that the 
microscopic forms floating in the water and cap- 
tured by our fine silk tow-nets included (1) the 
pearl oyster itself in its youngest free-swiraining 
stages, (2) its food, cot merely when young, but 
throughout life and (3) the young stages of the 
parasitic worm which causes pearl formation ; an 
that the larger forms at the bottom — sponges, 
corals, star fish, molluscs and fishes —are tlie all- 
important enemies or fellow competitors of the 
oyster (for food and attachiiient areas and growing 
room) which may ruin a bed either by their direc 
agressive uction or indirectly in the struggle for 
existence. We find that the 
CEYLON PEARL OYSTER IS DIOSCIOUS, 
or hFis the sexes separate, not only at any one 
period, but thionghout the life of the animal. Our 
experiments at Galle have shown that definitely; 
and theie ;ire no traces of the hermaphroditism. 
Emission of the generative products takes place 
directly into the surrounding water, where the ova 
are fertilised and consequently there is no retention 
of eggs or embryos within the body of the female. 
The male is stimulated to emit spermatozoa by 
the presence of ova in the surrounding water. 
There is no marked disproportion in numbers be. 
tween the sexes : out of a couple of hundred collec 
ted together, 87 were males, 71 females, and the 
remainder immature -or indeterminate. Kepro- 
duction appears to take place to some extent 
throughout the year, and stray individuals sexually 
ripe may be found at; any time ; but there are two 
maxima when the majority of the mature oysters 
reproduce, viz, in March to May and in October 
to November. Larval development takes place ia 
the surface waters of the sea, and from our obser- 
vations we draw the conclusion that the young 
animal may settle down as "spat" within five days 
of the fertilisation of the egg. At the same time, 
from the sizi3 of some of the larvte we have found, 
we consider it probable that the free-swimming 
period may on occasions be considerably prolonged. 
We were able to rear young larval stages in the 
Galle Marine Biological Station, and we caught 
the later ones in the tow-nets on the pearl banks. 
We found the youngest fixed spat on Zoophytes 
and Alg.'e early in November and early in March. 
All fixed stages from one similar to the latest of 
the free stages up to young oysters having the 
adult characteristics of shell we-e found during 
March and April attached to both rooted and float- 
ing Alg;c in !,various parts of the Gulf of Mannar. 
The so-called "false spat" (other sinaller species 
of Avicula) also occur on Zoophytes aud Alg?e ; but 
during the times of our investigations there was 
undoubtedly abundance of the true pearl oyster 
spat both on flamentous Algpe from the bottom 
and also on floating Sargassmn weed. The 
spat is actively locomotive. We have many 
observations showing the rapidity with which it 
can detach and re-attach itself, and the rate at 
which it can travel. There is no doubt that in 
this young stage the pearl oyster can leave the 
weed to which it first hxes and transfer to a coral 
or Nullipore fragment on the paar, or can move 
from an unsuitable spot in search of a better. Its 
tendency to climb upwards whenever shaken on 
to the floor of an aquarium is probably an indica- 
tion of an instinct to ascend any solid objects on 
the sea bottom, which must often save it from 
being smothered in the loose sand, 
OUR EXPERIMENTS AT THE GALLE LABORATORY 
have shown us that not only the young, but also 
the adult pearl-oyster is able to cast oft its old 
attachment, move to a new place, and there spin n 
new byssus, and this not once or twice, but repeat- 
edly, up to 7 times in li days, as our records show. 
We do not mean to assert that the oysters have 
