In examining the structure of the test, where it is pf no great thickness, we find a simple lamina, quite on the interior surface, consist ing of an epithelium, with polygonal cells, which is united to the mantle by scattered, crossing, muscular fibres. A thick layer of fibres suc ceeds, having cells and nuclei disseminated in it. The fibres themselves are colourless, un dulating, resembling the fibrils of the fibrous tissue of vertebrate animals, but narrower ; they measure 0.0002'"-0.0004'" in thick ness; they are never ramified, nor united into bundles. In their direction they are in part parallel to the axis of the animal, as in the interior lamina; ; and in part differently in terlaced, so that some are disposed longi tudinally, and others transversely (circular and longitudinal fibres) ; hence, from the disposition of the two kinds of layers, the test can very easily be split into, sometimes very delicate, !minx. There is apparently no intermediate substance accompanying the fibres, but the lamina: formed by their diver gences are occupied by a quantity of granules and vesicles of different forms. Firstly, there are minute colourless molecules, in some parts so abundant as to render the fibres in distinct, and to give to some of the thicker layers a finely granular aspect ; secondly, crystals, which are present only in the ex terior layers ; thirdly, nuclei, measuring 0.001' to 0 003m, often with large granules, apparently of fat ; fourthly, cells of different forms. Some of these cells, containing nuclei and brown Pigment-granules, are round, having a diameter of 0.005"/ to 0.0I"', or elon gated, with a diameter of 0•006" to 0.008'"; others are of a pale colour, and, from the double nuclei and the included cells (2 to 7) seen in some of them, strongly resemble the cartilage-cells of the superior animals. This resemblance is rendered the more strik ing by the round or elongate form of these cells, by the peculiar arrangement of the en closed cells, and by the union of some of the mother-cells, in this case generally round and smaller, into groups of two or four. This resemblance, however, is only external, and due only to the fact that the cells increase by formation in the interior, as in cartilage ; because more exact observations show that these cells, by further transitions, become identical with the simpler pigment cells described above, and are only the more developed forms of the latter, the pigment having disappeared by degrees on account of the condition of their growth.
The third layer is formed of a yellowish, horny epidermis. The thin hairs which cover the surface of the Cynthia papillata are formed by this and the exterior fibrous layer. At certain spots a bundle of fibres springs up from the plane surface of the latter, which, being coated with the horny epidermis, rises on the surface as small needle-shaped bodies.
Where the test of the Cynthia attains a thick ness of lr to 11"f and more, its composition often changes in a remarkable manner (fig. 776.). In this case the epithelium is succeeded by a clear, homogeneous, structureless mass, of a moderate thickness, with scattered pigment cells and nuclei. Next is a fine fibrous tissue, composed of a great number of thin layers of circular fibres, without cells or nuclei, and of radiating fibres that unite these layers ; this passes externally into an irregular fibrous tissue, covered with a horny epidermis. Where this peculiar stratification of fibres exists, the test is not separable into laminae, because the radiating fibres firmly connect the thin layers of circular fibres.
Treated with hydrochloric acid and with soda, the test of Cynthia is rendered quite white. The pigment-cells, the coloured epi dermis, the mother-cells, the crystals, the epitheliutn, the nuclei, and the granules are dissolved, and there remain only the fibres and the homogeneous substance that exists here and there. These two elements, then, are composed of cellulose.
The structure of the thick test of the Cynthia Canopus is very similar to the above. Interiorly there is an epithelium, then a thick layer of longitudinal and circular fibres, some what indistinctly stratified, in which, towards the exterior, crystals and largish round bodies, composed apparently of groups of cells, are disseminated ; lastly, a thin layer of solid, whitish epidermis, with little conical papilla, usually accompanied by processes from the fibrous layer. The thick fibrous layer, only, resists the action of hydrochloric acid and of soda ; all the rest is dissolved without any residue.
Cynthia pomaria presents, as the chief ma terial of its test, a layer of fibres similar to those previously described, having chiefly a longitudinal direction. Between the fibres are crystals, round pigment-cells, measuring to 0'006m, and further, here and there, peculiar elongated cells, filled with yellow granules, measuring 0.008"' and more. In ternally, and adhering to the fibrous layer, is a simple epithelium, with polygonal cells that have diameters of 0-006' to 0•008m. This adheres to the inner tunic by means of mus cular fibres. Externally the fibrous layer is covered by a yellowish, solid layer of unde termined structure. In the interior parts of the fibrous layer occurs a somewhat large number of peculiar cells, apparently not ana logous to any other of animal or vegetable structure. These cells are primitively similar to pigment-cells, and round, but possessing a thicker membrane, and without any apparent nucleus. Subsequently they grow, preserving their shape, to the size of The mem brane at the same time continues to thicken, so much so that the cavity of the cell increas ing but slightly, the membrane attains a thick ness of Lastly, the size of the cell increases to 0.02"/ and the thickness of the membrane to 0'006m. Whilst this develop ment is going on, fine lines are observable in the thickened cell-membrane, and ultimately the membrane is transformed into fibres, so that one may see the moderately sized cells in their cavities, and yet occupied by pigment or by pale granules, enveloped with an elegant skein of fine, cylindrical, opaque fibres, which can be isolated by compression. Whether the fibres have a spiral arrangement is uncertain. MM. Lewig and Milker, who discovered these remarkable fibre-coated cells, remark that, as to the manner in which this curious trans formation of the pigment-cells is brought about, the increase of thickness in the cell membrane would take place by growth, or by a development similar to that which occurs in many vegetable cells, the membrane of which consists of several layers, or by the de position of a substance applied externally. As there is no appearance of any internal or external deposit, and as the cell-cavity is not diminished during the thickening of its walls, they consider the first to be the more likely cause of the above conditions, but, neverthe less, not sufficient alone to account for the circumstances. It is difficult, perhaps, to say how these fibres are formed, but it may be that the cell-walls, by partial solidification, ultimately become separable into fibres.