Natural History of Molluscous Animals. 39 
Art. X. An Introduction to the Natural History of Molluscous 
Animals. Ina Series of Letters. By G. J 
Letter 3. Indirect and Direct Benefits. 
Sir, 
In my last letter I illustrated, at some length, one grand 
use of the Molldsca in the economy of nature: that, viz., of 
furnishing sustenance to many animals. But you will observe 
that a vast number of the class are themselves carnivorous, 
and become thus a means, in the hand of Providence, of keep- 
ing in check the multiplication of the tribes on which they 
prey, and of preserving between them that due proportion and 
** balance of power” which is as necessary in the animal, as in 
the political, world. Others, again, are gifted with the remark- 
able property of boring through stone and wood, and thus 
reduce to dust the rock over which the waves might have 
broken in vain, and remove those forests which the torrents 
and tornadoes of tropical climes annually float to the sea. In 
this sense, even the “ fell Terédo” ministers to good. ‘ The 
seaman,” to adopt the rather pompous language of a very ex- 
cellent author, ‘as he beholds the ruin before him, vents his 
spleen against the little tribes that have produced it, and 
denounces them as the most mischievous vermin in the ocean. 
But a tornado arises, the strength of the whirlwind is abroad, 
the clouds pour down a deluge over the mountains, and 
whole forests fall prostrate before its fury. Down rolls the 
gathering wreck towards the deep, and blocks up the mouth 
of that very creek the seaman has entered, and where he now 
finds himself in a state of captivity. How shall he extricate 
himself from his imprisonment? an imprisonment as rigid as 
that of the Baltic in the winter season. But the hosts of the 
Zerédo are in motion: thousands of little augers are applied 
to the floating barrier, and attack it in every direction. It is 
perforated, it is lightened, it becomes weak ; it is dispersed, or 
precipitated to the bottom; and what man could not effect, is 
the work of a worm. ‘Thus it is that nothing is made in vain ; 
and that, in physics, as well as in morals, although evil is 
intermingled with good, the good ever maintains a predomi- 
nancy.” 
The conversion, through their agency, of other materials 
into lime, seems, however, to be the great purport of the cre- 
ation of molluscous atin ale Shells consist of carbonate of 
lime with a greater or less proportion of animal matter, and 
the animals form these shells from their food, which contains 
* Good’s Book of Nature, vol. 1. p. 265. 
D 4 
