104 
THE PEARL OYSTER 
migratory and therefore cease to require the provision of 
a portable commissariat store. And what is the age at 
which this change of condition supervenes may seemingly 
be discerned from the presence or absence of the spines 
and foliations. I note that in the first year they begin 
to disappear from the region of the umbo ; but they were 
plentiful on the remainder of the shell of oysters said to be 
18 months old, and I saw no live ones older than that. 
On the other hand, however, the reason for the spines 
and foliations with the vegetable growth and insect life 
reared thereon being continued near the margin of the 
shell after disappearance from the umbo may easily be sup- 
posed to be that only near margin could it be of direct 
use to the oyster, because only from near the margin could 
food be drawn into the mouth of the oyster by the current 
which it creates for the purpose. 
If pearl oysters carry on their two sides thickets of algse 
affording, with the formation of the shell, harbour for 
diatoms, animalcules, &c., and the harbour increases with the 
growth of the animal, and a diatom frustule produces a 
thousand millions in a month, the pearl oyster should not 
have to compete, as the English oyster is said to do, for 
food, but should have always an ample store if only the 
water will remain clear enough for it to be continuously 
feeding. But possibly varying depths affect the growth 
and fertility of the algae and diatoms — that is a further 
question. For the English oyster tides aie said to be useful 
in bringing food to the foreshores ; but our Indian pearl 
oyster grows his own food and only wants still clear water 
wherein to feast upon it. The abundance of food seems to 
be further deducible from the fact that we find the pearl 
oyster not in a single layer, but frequently piled up one over 
another, the lowest clinging by its byssus to the coral rock 
or other hard formation and the one above it clinging to 
the shell of the one below it, and another clinging similarly 
