SECRETIONS OF THE SKIN. 16) 
As analogous to shells, as to their use in the animal eco- 
nomy, we mention the membranaceous sheaths in which the 
bodies of several of the annelides reside, and the tubular 
coverings, some of which are membranaceous, others calca- 
reous, which protect the zoophytes. 
The earthy matter of shells is ime, im union with car- 
bonic acid. Phosphate of lime has likewise been detected, 
but in small quantity. The colour is secreted from the 
animal, along with the matter-of the shell. 
6. Crusts.—These are, in general, more brittle in their 
texture than shell. They exhibit remarkable duferences as 
to thickness and composition. ‘They differ from shells 
chiefly in containing a considerable portion of phosphate of 
lime, and in a greater subdivision of parts. In some cases, 
however, as the crusts of the bodies of insects, the earthy 
matter is almost absent, and they may be regarded as form- 
ed of cuticle alone. Where they contain much earthy mat- 
ter, as in the crusts of lobsters, the epidermis may be de- 
tected as a cover, and the corium beneath may be perceived 
as a very thin film. In many cases, these crusts are re- 
newed periodically ; and, in all, they are readily repaired. 
Crusts occur in insects, the crustacea, and the echineder- 
mata, or sea-urchins, and star-fish. 
In examining these different appendices of the skin, we 
perceive that they pass, by insensible degrees, into one an- 
other, as hair into spines, horns into nails, scales into shells, 
and crusts into membranes. They have all one common 
origin, namely, the skin; and, independent of secondary 
purposes, they all serve for protection. 
III.—SEcRETIONS OF THE SKIN. 
Besides these appendices of the skin, which we have enu- 
merated, it yet remains that-we take notice of some of its 
