UNIVALVES. 157 
was necessary, both light and thin, is completely 
impervious either to moisture or to air.” The young 
Helix, like his numerous brethren of the family of 
Univalve, emerges into life with a covering adapted 
to his exigences, and this enlarges with his growth, 
by means of a certain viscous exudation from innu- 
merable pores. Now the aptness of this secretion 
to the purpose for which it is designed, its pro- 
perty of congealing into a firm and hard substance, 
independent of any effort on the part of the inhabi- 
tant, cannot be referred, as the same admirable writer 
has observed, to any other cause than express design ; 
and that, not on the part of the unconscious architect, 
who, although he might build the house, could not 
supply the materials. Moreover, the form of the 
building, with its pillar and convolution, is not only 
a very artificial one, but admirably adapted to the 
exigences of the inhabitant; which is, confessedly, 
one of the most feeble and unprovided of all artificers. 
Nay more, the testaceous coverings of such as live on 
land, or inhabit ponds and ditches, are scarcely able 
to resist the slightest pressure; while in others, their 
defensive strength suits well with the lives of those 
that have to sustain the dangers of a stormy element, 
and a rocky bottom, as well as the attacks of vora- 
cious fish. The sealing up of the mouth, which 
serves in several species as an effectual protection 
