The building up of the body by the repetition and the modification of a few similar parts, which is so obvious from the study of the general form of the somites and of their appendages, is still more remarkably illustrated, if we pursue our investigations further, and trace {175} out the more intimate structure of these parts. The tough, outer coat, which has been termed the cuticula, except so far as it presents different degrees of hardness, from the presence or absence of calcareous salts, is obviously everywhere of the same nature; and, by macerating a crayfish in caustic alkali, which destroys all its other components of the body, it will be readily enough seen that a continuation of the cuticular layer passes in at the mouth and the vent, and lines the alimentary canal; furthermore, that processes of the cuticle covering various parts of the trunk and limbs extend inwards, and afford surfaces of attachment to the muscles, as the apodemata and tendons. In technical language, the cuticular substance which thus enters so largely into the composition of the bodily fabric of the crayfish is called a tissue.
The flesh, or muscle, is another kind of tissue, which is readily enough distinguished from cuticular tissue by the naked eye; but, for a complete discrimination of all the different tissues, recourse must be had to the microscope, the application of which to the study of the ultimate optical characters of the morphological constituents of the body has given rise to that branch of morphology which is known as Histology.
FIG. 49.—Astacus fluviatilis.—The corpuscles of the blood, highly magnified. 1–8, show the changes undergone by a single corpuscle during a quarter of an hour; n, the nucleus; 9 and 10 are corpuscles killed by magenta, and having the nucleus deeply stained by the colouring matter.
If we count every formed element of the body, which is separable from the rest by definite characters, as a tissue, there are no more than eight kinds of such tissues in the crayfish; that is to say, every solid constituent {176} of the body consists of one or more of the following eight histological groups:—
1. Blood corpuscles; 2. Epithelium; 3. Connective tissue; 4. Muscle; 5. Nerve; 6. Ova; 7. Spermatozoa; 8. Cuticle.
1. A drop of freshly-drawn blood of the crayfish contains multitudes of small particles, the blood corpuscles, which rarely exceed 1‐700th, and usually are about 1‐1000th, of an inch in diameter (fig. [49]). They are sometimes pale and delicate, but generally more or less dark, from containing a number of minute strongly refracting granules, and they are ordinarily exceedingly irregular in form. If one of them is watched {177} continuously for two or three minutes, its shape will be seen to undergo the constant but slow changes to which passing reference has already been made (p. [69]). One or other of the irregular prolongations will be drawn in, and another thrown out elsewhere. The corpuscle, in fact, has an inherent contractility, like one of those low organisms, known as an Amœba, whence its motions are frequently called amœbiform. In its interior, an ill-marked oval contour may be seen, indicating the presence of a spheroidal body, about 1‐2000th of an inch in diameter, which is the nucleus of the corpuscle (n). The addition of some re-agents, such as dilute acetic acid, causes the corpuscles at once to assume a spherical shape, and renders the nucleus very conspicuous (fig. [49], 9 and 10). The blood corpuscle is, in fact, a simple nucleated cell, composed of a contractile protoplasmic mass, investing a nucleus; it is suspended freely in the blood; and, though as much a part of the crayfish organism as any other of its histological elements, leads a quasi-independent existence in that fluid.
2. Under the general name of epithelium, may be included a form of tissue, which everywhere underlies the exoskeleton (where it corresponds with the epidermis of the higher animals), and the cuticular lining of the alimentary canal, extending thence into the hepatic cæca. It is further met with in the generative organs, and in the green gland. Where it forms the subcuticular layer of the integument and of the alimentary canal, it is found to {178} consist of a protoplasmic substance (fig. [50]), in which close set nuclei (n) are imbedded. If a number of blood corpuscles could be supposed to be closely aggregated together into a continuous sheet, they would give rise to such a structure as this; and there can be no doubt that it really is an aggregate of nucleated cells, though the limits between the individual cells are rarely visible in the fresh state. In the liver, however, the cells grow, and become detached from one another in the wider and lower parts of the cæca, and their essential nature is thus obvious.
