SECRETIONS OF THE SKIN. 



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 lime, in 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 differences 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 echinoder- 

 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 



