THE PINNA. 
349 
which hangs a stone weighing abont thirty pounds, and after taking a long breath is swiftly 
carried to the bottom. He then flings himself on his face, fills his bag as fast as he can, and 
when his breath begins to fail, shakes his rope as a signal, and is drawn np together with the 
bag. 
Very exaggerated accounts have been given of the time passed unaer water by the divers, 
from two to seven minutes being mentioned as the usual periods. The real fact, however, is, 
that one minute is the ordinary average ; a few men being able to endure an immersion of a 
minute and a half. This is a long period, as any one will confess who has attempted to repeat 
the feat. Yet, with a little practice, it can be achieved, even by those who can lay no claim 
to extraordinarily capacious lungs ; and I have more than once performed it with tolerable 
PEARL OYSTER.— Meleagrina margaritifera. 
ease. If the lungs be thoroughly filled four or five times in succession, and emptied to the last 
gasp, so as to expel all foul air that may be lingering in the tiny vesicles, the blood becomes 
so well oxygenized, that a further supply of breath will not be needed for some time, and a 
deep inspiration will serve to keep the blood in a healthy state for a marvellously long time. 
All swimmers who are fond of diving will find that they can remain under water nearly twice 
their usual time by taking this simple precaution. 
The best plan for procuring the Pearl Oyster is evidently the employment of the diving 
bell, so that the best shells might be leisurely selected, the spot left undisturbed, and the 
sharks outwitted. In the illustration are given specimens of the shell in various stages, as 
well as the interior, showing the pearls as they appear when the animal is removed. 
A laeoe, flattish, wedge-shaped shell, generally moored to a stone, or fastened to the 
bottom by a number of short threads, is the Pinna, so called from the Latin word, signifying 
a wing. 
The aggregate mass of these threads is termed the byssus, and is, indeed, a very curious 
object. The threads are spun by the foot, and are attached to the centre of each valve, thus 
forming a powerful cable by which the shell is moored to the rock. The threads are wonder- 
