404 PHILOSOPHY OF ZOOLOGY. 



structure. Those of the second kind, consisting of layers 

 regularly superimposed, have consequently a laminated 

 structure ; but between the k two kinds there are numerous 

 intermediate links, formed by a combination of the two pro- 

 cesses. 



In some cases, the hard parts of the skin are not entitled 

 to the appellation of Shell, but may rather be considered as 

 Horn. Such are the coverings of the mandibles of the Cut- 

 tle-fish, the branchial lid of the Aplysia, and the opercu- 

 lum of the Welk. The two last appendices, however, 

 though horny in some species are shelly in others. 



The position of the shell, with respect to the constituent 

 layers of the integuments, exhibits very remarkable dif- 

 ferences. In some it appears instead of a cuticle, or at least 

 without an external membrane investing it. In general, 

 however, it occurs between the cuticle and the skin ; a 

 position which induces CUVIER* to consider it as analogous 

 to the mucous web of the vertebral animals. Its intimate con- 

 nection with the muscular system of the animal, and the pro- 

 tection which it affords, seem adverse to such a conclusion. In 

 many species the testaceous substance occurs in folds of the 

 corium, or inserted in its substance. In this position it never 

 acquires the solid texture which shells exposed, or covered 

 only by the cuticle exhibit. Those which are thus con- 

 cealed are in general white ; those which are more exposed 

 are frequently coloured. The colouring, however, does 

 not depend on the direct exposure to the light, as some 

 have imagined, for many shells which are destitute of a cu- 

 ticle are white, while many of those covered with a dense 

 cuticle are finely variegated beneath. 



Between the skin and the shell neither vessels nor nerves 

 have been traced ; and the manner in which the latter is 



* Lec.'d*An. Comp. xiv. 11. 



