FIG. 50.—Astacus fluviatilis.—Epithelium, from the epidermic layer subjacent to the cuticle, highly magnified. A, in vertical section; B, from the surface. n, nuclei.
3. Immediately beneath the epithelial layer follows a tissue, disposed in bands or sheets, which extend to the subjacent parts, invest them, and connect one with another. Hence this is called connective tissue.
FIG. 51.—Astacus fluviatilis.—Connective tissue; A, second form; B, third form. a, cavities; n, nuclei. Highly magnified.
The connective tissue presents itself under three forms. In the first there is a transparent homogeneous-looking matrix, or ground substance, through which are scattered many nuclei. In fact, this form of connective tissue {179} very closely resembles the epithelial tissue, except that the intervals between the nuclei are wider, and that the substance in which they are imbedded cannot be broken up into a separate cell-body for each nucleus. In the second form (fig. [51], A) the matrix exhibits fine wavy parallel lines, as if it were marked out into imperfect fibres. In this form, as in the next to be described, more or less spherical cavities, which contain a clear fluid, are excavated in the matrix; and the number of {180} these is sometimes so great, that the matrix is proportionally very much reduced, and the structure acquires a close superficial similarity to that of the parenchyma of plants. This is still more the case with a third form, in which the matrix itself is marked off into elongated or rounded masses, each of which has a nucleus in its interior (fig. [51], B). Under one form or another, the connective tissue extends throughout the body, ensheathing the various organs, and forming the walls of the blood sinuses.
The third form is particularly abundant in the outer investment of the heart, the arteries, the alimentary canal, and the nervous centres. About the cerebral and anterior thoracic ganglia, and on the exterior of the heart, it usually contains more or less fatty matter. In these regions, many of the nuclei, in fact, are hidden by the accumulation round them of granules of various sizes, some of which are composed of fat, while others consist of a proteinaceous material. These aggregates of granules are usually spheroidal; and, with the matrix in which they are imbedded and the nucleus which they surround, they are often readily detached when a portion of the connective tissue is teased out, and are then known as fat cells. From what has been said respecting the distribution of the connective tissue, it is obvious that if all the other tissues could be removed, this tissue would form a continuous whole, and represent a sort of model, or cast, of the whole body of the crayfish. {181}
FIG. 52.—Astacus fluviatilis.—A, a single muscular fibre, transverse diameter 1‐110th of an inch; B, a portion of the same more highly magnified; C, a smaller portion treated with alcohol and acetic acid still more highly magnified; D and E, the splitting up of a part of a fibre, treated with picro-carmine, into fibrillæ; F, the connection of a nervous with a muscular fibre which has been treated with alcohol and acetic acid. a, darker, and b, clearer portions of the fibrillæ; n, nuclei; nv, nerve fibre; s, sarcolemma; t, tendon; 1–5, successive dark granular striæ answering to the granular portions, a, of each fibrilla.
4. The muscular tissue of the crayfish always has the form of bands or fibres, of very various thickness, marked, when viewed by transmitted light, by alternate darker and {182} lighter striæ, transversely to the axis of the fibres (fig. [52] A). The distance of the transverse striæ from one another varies with the condition of the muscle, from 1‐4,000th of an inch in the quiescent state to as little as 1‐30,000th of an inch in that of extreme contraction. The more delicate muscular fibres, like those of the heart and those of the intestine, are imbedded in the connective tissue of the organ, but have no special sheaths. The fibres which make up the more conspicuous muscles of the trunk and limbs, on the other hand, are much larger, and are invested by a thin, transparent, structureless sheath, which is termed the sarcolemma. Nuclei are scattered, at intervals, through the striated substance of the muscle; and, in the larger muscular fibres, a layer of nucleated protoplasm lies between the sarcolemma and the striated muscle substance.
