8 CONCHOLOGIST’S COMPANION. 
agent.* The same admirable economy, which gave 
rise to the arrangement of little prisms in the mem- 
branaceous part of different shells, is also apparent in 
their configuration, notwithstanding the various deco- 
rations with which they are frequently embellished. 
This curious fact is strikingly exemplified in the 
spiral form of many of these elegant receptacles. 
Round or oblong cases would effectually defend the 
animal inhabitants from external injury; but these 
require in their construction a considerable quantity 
of the viscous exudation already noticed ; whereas, 
a thin coating, fitted to the shape of each individual, 
answers the purpose equally as well, and enables 
it to feel the vibrations of the aqueous element in 
which it frequently resides. 
Thus, in subserviency to this general system of 
economy, as well as in accordance with the instincts 
of the animals themselves, we find that different 
species of shell-fish are inclosed in various kinds of 
testaceous coverings. Those which defend the active 
family of Donax; enable them to dart away on the 
approach of danger; while the shells of their less 
active relatives, the perambulating Razor-sheaths, 
are admirably adapted to assist their movements 
through the yielding sand. The warlike Chitones, 
walk abroad in coats of mail closely fitted to their 
* Burrows. 
