The chemical stability of chitin is so remarkable that we might well expect it to accumulate like the inorganic constituents of animal skeletons, and form permanent deposits. Schlossberger[33] has, however, shown that it changes slowly under the action of water. Chitin kept for a year under water partially dissolved, turned into a slimy mass, and gave off a peculiar smell. This looks as if it were liable to putrefaction. The minute proportion of nitrogen in its composition may explain the complete disappearance of chitin in nature. [33] Ann. d. Chem. u. Pharm., Bd. 98. _The Chitinous Cuticle._ The chitinous exoskeleton is rather an exudation than a true tissue. It is not made up of cells, but of many superposed laminæ, secreted by an underlying epithelium, or “chitinogenous layer.” This consists of a single layer of flattened cells, resting upon a basement membrane. A cross-section of the chitinous layer, or “cuticle,” examined with a high power shows extremely close and fine lines perpendicular to the laminæ. The cells commonly form a mosaic pattern, as if altered in shape by mutual pressure. The free surface of the integument of the Cockroach is divided into polygonal, raised spaces. Here and there an unusually long, flask-shaped, epithelial cell projects through the cuticle, and forms for itself an elongate chitinous sheath, commonly articulated at the base; such hollow sheaths form the hairs or setæ of Insects--structures quite different histologically from the hairs of Vertebrates. [Illustration: Fig. 8.--Diagram of Insect integument, in section. _bm_, basement membrane; _hyp_, hypodermis, or chitinogenous layer; _ct_, _ct_′, chitinous cuticle; _s_, a seta.] The polygonal areas of the cuticle correspond each to a chitinogenous cell. Larger areas, around which the surrounding ones are radiately grouped, are discerned at intervals, and these carry hairs, or give attachment to muscular fibres. Viallanes (loc. cit.) has added some interesting details to what was previously known of Insect-hairs. There are, he points out, two kinds of hairs, distinguished by their size and structure. The smaller spring from the boundary between contiguous polygonal areas, and have no sensory character. The larger ones occupy unusually large areas, surmount chitinogenous cells of corresponding size, and receive a special nervous supply. The nerve[34] expands at the base of the hair into a spindle-shaped, nucleated mass (bipolar ganglion-cell), from which issues a filament which traverses the axis of the hair, piercing the chitinogenous cell, whose protoplasm surrounds it with a sheath which is continued to the tip of the hair. Such sensory hairs are abundant in parts which are endowed with special sensibility. [34] Previously observed by Leydig in _Corethra_. [Illustration: Fig. 9.--Nerve-ending in skin of Stratiomys larva. _h_, hairs; _b_, their chitinous base; _c_, nucleus of generating cell; _g_, ganglion cell. × 250. Copied from Viallanes.]