[Illustration: 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.