. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. Cells of pigmentary layer of shell of Crab; a, papil- lary elevation of subjacent layer. Magnified 400 diameters. then obvious that the nearly colourless poly- gonal reticulations are the thickened ivalls of cells, each of them being divided by a distinct line, which marks the junction of the conti- guous boundaries ; whilst the dark spaces or areolas are the cavities of the cells, filled with colouring matter, or with some other semi- opaque substance. This cellular layer is not uniformly disposed over the entire surfac
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. Cells of pigmentary layer of shell of Crab; a, papil- lary elevation of subjacent layer. Magnified 400 diameters. then obvious that the nearly colourless poly- gonal reticulations are the thickened ivalls of cells, each of them being divided by a distinct line, which marks the junction of the conti- guous boundaries ; whilst the dark spaces or areolas are the cavities of the cells, filled with colouring matter, or with some other semi- opaque substance. This cellular layer is not uniformly disposed over the entire surface of the crab-shell; for the calcareous layer beneath rises up througli it in little papillary eleva- tions (fig- 425.«), to the summit of which the epidermis adheres. It is from the deficiency of the pigmentary layer at these points, that the shell derives its minutely speckled appear- ance. The internal layer is that which constitutes by far the thickest part of the shell of the crab, and which must be regarded as its fun- damental or essential element, since (according to M. Lavalle) it is never wanting in the Decapod Crustacea, whilst other layers are sometimes deficient. It is in this internal layer, that the calcareous matter is chiefly de- posited ; but even after this has been re- moved, a very distinct animal basis is left, possessing considerable firmness, and closely resembling that which is left after the decal- cification of dentine. When a thin section of it is made parallel to its surface, and sub- jected to a high magnifying power, it is seen to be composed of an apparently homogeneous substance, studded with minute points, each surrounded by a clear space, which correspond with those seen in a section of dentine cut at right angles to the course of its tubuli, anil which would seem to possess the same essen- tial character with them. A thin section of the shell taken in the opposite direction (i. e. from surface to surface) leaves no doubt, when examined with
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