An integument of one or other of these descriptions is to be met with in all the Sertularian and Actinoid Polypes, and is obviously, in these cases, the result of a process of excretion. In the Illedusce and Beroidce, on the other hand, where the integu ment is thick and gelatinous, the ecderonic tissue is converted, as a whole, into what closely resembles rudimentary connective tissue, in which elastic elements and muscular fibres are developed. The presence of peculiar organs, called the " Thread or Urticating cells," constitutes an extremely characteristic feature in the integument of these creatures. These (fig. 311.) are composed of a delicate mem branous sac (a), enclosing a much thicker one (b), which is open at one extremity, the aperture being stopped by the end of a more or less irregular short stiff sheath (c), some thnes giving attachment to several distinct rays or spines (d), applied together, which is fixed to the edges of the aperture, and oc cupies the axis of the inner sac. To the ex tremity of this sheath a long, frequently toothed filament is attached (e), and lies coiled up round the central sheath, and in close contact with the walls of the sac. The latter are very elastic, and seeno to be tensely stretched by the contained fluid during life ; for, on pressure, the sac suddenly bursts, and its contents are evacuated so rapidly as hardly to allow of the process being traced. I be lieve, however, that the long filament is pushed out by the side or through the axis of the central sheath, remaining still firmly attached to the latter, so that the result is the appear ance exhibited in the accompanying figure (c), where the sac is seen empty, the long serrated filament being attached to the sheath, which, everted and with its spines spread out, is itself fixed to the margins of the aperture. The violent protrusions of these minute serrated filaments, aided, perhaps, by some aridity of the liquid of the sac, is in the larger kinds, such as those which exist in Physalia, exceed ingly irritating to the human skin, and usually proves fatal to the minute creatures on which the Hydrozoic and Anthozoic polypes prey.
Integument of the Annulosa.—The integu ment of the lower Annulose tribes, of young forms and of the more delicate parts of a great majority of the higher Annulosa, con sists of a thin structureless chitinous mem brane developed from the subjacent cellular ecderon, in a manner essentially similar to what has been described in the Polypes.
Leydig has particularly described this form of integument in Entomostracous Crustaceans, (Branchipus and Argulus) in insect larvw, (Corethra), and among the Annelids in Pisci cola, Nephelis, Sanguisuga, Clep sine and Lumbricus, where the integument consists of two portions—a deep cellular layer and a superficial lay er, which is either abso lutely structureless, or is fibrillated ; being in no case fortned by the coalescence of the sub jacent cells, but by excretion from them.
A similar structureless excreted integument is found also in Planari, Nemertidx, in many Cestoidia, Nematoidea and Trematoda, and, according to the late researches of Leydig, on Synapta, in the Echinoderms also. Where
the integument is not very thin, and con sists of several layers of chitinous matter, the added laminx commonly take on a fibrous structure. The Nematoid worms present par ticularly good examples of this complication. Thus, for instance, the integument of Mermis albicans, which has lately been examined with much care by Dr. Meissner, consists of three layers, the middle of which is double. The outermost of these layers is either structureless or presents a distinction into transverse hex agonal plates, each of which occupies of the circumference of the animal. At the head and tail, small polygonal plate-like markings replace these, and such small plates could be detected, making up the large ones. Dr. Meissner calls them " cells," but expressly states that he never detected any nucleus in them, and it seems more probable that they are produced by modifications of the original external structureless layer, similar to those which, as will be seen, occur in the Crustacea and Mollusca.
The middle substance of this integument is composed of two layers of fibres one above the other. The fibres are parallel in the same laver, but those of the two layers cross one another at right angles, so that they form two sets of opposite spirals. The fibres are sharply contoured, dense, and brittle, and those of each layer are divided into six sets, corre sponding with the six sections of the body. At the sutures the fibres of each bend back upon themselves, and run in a parallel course to the opposite suture.
The deep layer is the thickest ; it appears longitudinally striated on section, and may be split into lamellce of any thickness ; other wise it is perfectly structureless.
In the Nemertidm, according to the re searches of Quatrefages, the integument has essentially the same structure, consisting of a superficial structureless ciliated lamina, with deeper vacuolated and fibrillated layers. In the other Turbellaria the vacuolated struc ture is predominant.
This fibrous chitinous integument is still better developed in the Insecta.
According to Mayer (/. c.) the chitinous integument of Lucanus cervus is composed of glassy rods with sharply defined dark, paral lel edges, which by their mutual apposition and anastoinosis, and probably by the interposition of a connecting mass, form thin layers. The rods in each layer are parallel, but those of different layers cross one another at angles of from 45° to 90°; so that a horizontal section presents a sort of elegant cross-hatching, the lines of which are about 0.008 mm. apart. The outer surface of this laminated mass is invested by a transparent homogeneous sub stance containing pigment, and above this by a layer of epidermic " cells " (0'005 to 0'01 mm. in diameter), with'nuclei and nucleoli, their edges being separated by an intermediate substance. Internally, there is also a layer of epidermic " cells " which are polygonal from mutual pressure. They are without nuclei, but possess a short spine, arising from the centre of the cell, and ending by a sharp point. Quekett (/. c.) describes a similar structure, consisting of striated larninm, in the integu ment of Dynastes Hercules.