MOLLUSKS, 351 
animals, which have various modes of getting at it, in 
spite of its shelly defense. From some of these it can 
secure itself by closing its valves as soon as it is alarmed ; 
and against others it has a more active means of defense 
in the violent expulsion of the water included between 
them, which (as it is itself fixed) will frequently drive off 
its opponent. Various animals attack it, also, by perfo- 
rating its shell; and to these, also, it can offer a passive 
resistance, by depositing new shelly matter within. So 
that even this lowly-organized being, commonly regard- 
ed as one of the most vegetative of animals, is provided 
by its Creator with such means as are necessary for its 
preservation, and doubtless, also, for its enjoyment.” 
569. Pearl Oysters, from which pearls are obtained, 
are found both in the Old and New World. Ceylon is 
famous for its pearl fisheries. Pearls are globules of 
** nacre,” which chances to be deposited in this form, in- 
stead of being spread out over the inner surface of the 
shell; it being in the latter case called mother-of-pearl. 
The Pearl Oyster is not the only animal from which 
pearls can be obtained. They are often found in other 
shells. 
570. The Pectens, or Scallops, of which a species is 
given in Figure 262, are distin- 
guished by the regular ribs of 
the shell, and by the two angu- 
lar projections that widen the 
sides of the hinge. They have 
a small foot, and some species 
have a byssus. In some the 
shell is beautifully colored. 
571. Among those Conchif- 
erous Acephala that have si- 
phons are the Clam-shells, the 
Cockles, etc. Among the Clam- 
shells is one which is the largest known Mollusk. It is 
the Tridacna, or Giant Clam-shell, found only in the In- 
Fig. 262.—Scallop. 
