[1] If crayfish are not to be had, a lobster will be found to answer to the description of the former, in almost all points; but the gills and the abdominal appendages present differences; and the last thoracic somite is united with the rest in the lobster. (_See_ Chap. V.) * * * * * It is a matter of common information that a number of our streams and rivulets harbour small animals, rarely more than three or four inches long, which are very similar to little lobsters, except that they are usually of a dull, greenish or brownish colour, generally diversified with pale yellow on the under side of the body, and sometimes with red on the limbs. In rare cases, their {6} general hue may be red or blue. These are “crayfishes,” and they cannot possibly be mistaken for any other inhabitants of our fresh waters. [Illustration: FIG. 1.—_Astacus fluviatilis._—Side view of a male specimen (nat. size):—_bg_, branchiostegite; _cg_, cervical groove; _r_, rostrum; _t_, telson.—1, eye-stalk; 2, antennule; 3, antenna; 9, external maxillipede; 10, forceps; 14, last ambulatory leg; 17, third abdominal appendage; 20, lateral lobe of the tail-fin, or sixth abdominal appendage; XV, the first; and XX, the last abdominal somite. In this and in succeeding figures the numbers of the somites are given in Roman, those of the appendages in ordinary numerals.] The animals may be seen walking along the bottom of the shallow waters which they prefer, by means of four pairs of jointed legs (fig. 1); but, if alarmed, they swim {7} backwards with rapid jerks, propelled by the strokes of a broad, fan-shaped flipper, which terminates the hinder end of the body (fig. 1, _t_, _20_). In front of the four pairs of legs, which are used in walking, there is a pair of limbs of a much more massive character, each of which ends in two claws disposed in such a manner as to constitute a powerful pincer (fig. 1; _10_). These claws are the chief weapons of offence and defence of the crayfish, and those who handle them incautiously will discover that their grip is by no means to be despised, and indicates a good deal of disposable energy. A sort of shield covers the front part of the body, and ends in a sharp projecting spine in the middle line (_r_). On each side of this is an eye, mounted on a movable stalk (_1_), which can be turned in any direction: behind the eyes follow two pairs of feelers; in one of these, the feeler ends in two, short, jointed filaments (_2_); while, in the other, it terminates in a single, many-jointed filament, like a whip-lash, which is more than half the length of the body (_3_). Sometimes turned backwards, sometimes sweeping forwards, these long feelers continually explore a considerable area around the body of the crayfish.