[13] The dimensions of crayfishes at successive ages given at p. [31], beginning at the words “By the end of the year,” refer to the “écrevisse à pieds rouges” of France; not to the English crayfish, which is considerably smaller. Doubtless, the proportional rate of increment is much the same, in the two kinds; but in the English crayfish it has not been actually ascertained.
The distance from the orbit to the posterior margin of the carapace is nearly equal to that from the posterior margin of the carapace to the base of the telson, when the abdomen is fully extended, but this measurement of the carapace is commonly greater than that of the abdomen in the males and less in the females.
The general contour of the carapace (fig. [61]), without the rostrum, is that of an oval, truncated at the ends: the anterior end being narrower than the posterior. Its surface is evenly arched from side to side. The greatest breadth of the carapace lies midway between the cervical groove and its posterior edge. Its greatest vertical depth is on a level with the transverse portion of the cervical groove.
The length of the rostrum, measured from the orbit {232} to its extremity, is greater than half the distance from the orbit to the cervical groove. It is trihedral in section, and its free end is slightly curved upwards (fig. [41]). It gradually becomes narrower for about three-fourths of its whole length. At this point it has rather less than half the width which it has at its base (fig. [61], A); and its raised, granular and sometimes distinctly serrated margins are produced into two obliquely directed spines, one on each side. Beyond these, the rostrum rapidly narrows to a fine point; and this part of the rostrum is equal in length to the width between the two spines.
The tergal surface of the rostrum is flattened and slightly excavated from side to side, except in its anterior half, where it presents a granular or finely serrated median ridge, which gradually passes into a low elevation in the posterior half, and, as such, may generally be traced on to the cephalic region of the carapace. The inclined sides of the rostrum meet ventrally in a sharp edge, convex from before backwards; the posterior half of this edge gives rise to a small, usually bifurcated, spine, which descends between the eye-stalks (fig. [41]). The raised and granulated lateral margins of the rostrum are continued back on to the carapace for a short distance, as two linear ridges (fig. [61], A). Parallel with each of these ridges, and close to it, there is another longitudinal elevation (a, b), the anterior end of which is raised into a prominent spine (a), which is situated immediately behind the orbit, and may, therefore, be termed the post-orbital {233} spine. The elevation itself may be distinguished as the post-orbital ridge. The flattened surface of this ridge is marked by a longitudinal depression or groove. The {234} posterior end of the ridge passes into a somewhat broader and less marked elevation, the hinder end of which turns inwards, and then comes to an end at a point midway between the orbit and the cervical groove. Generally this hinder elevation appears like a mere continuation of the post-orbital ridge; but, sometimes, the two are separated by a distinct depression. I have never seen any prominent spine upon the posterior elevation, though it is sometimes minutely spinulose. The post-orbital ridges of each side, viewed together, give rise to a characteristic lyrate mark upon the cephalic region of the carapace.
FIG. 61.—A, D, & G, Astacus torrentium; B, E, & H, A. nobilis; C, F, & I, A. nigrescens (nat. size). A–C, Dorsal views of carapace; D–F, side views of third abdominal somites; G–I, Dorsal views of telson. a, b, post-orbital ridge and spines; c, branchio-cardiac grooves inclosing the areola.
A faintly marked, curved, linear depression runs from the hinder end of the post-orbital ridge, at first directly downwards, and then curves backwards to the cervical groove. It corresponds with the anterior and inferior boundary of the attachment of the adductor muscle of the mandible.
Below the level of this, and immediately behind the cervical groove, there are usually three spines, arranged in a series, which follow the cervical groove. The points of all are directed obliquely forwards, and the lowest is the largest. Sometimes there is only one prominent spine, with one or two very small ones; sometimes there are as many as five of these cervical spines.
The cardiac region is marked out by two grooves which run backwards from the cervical groove (fig. [61], A, c), and terminate at a considerable distance from the posterior {235} edge of the carapace. Each groove runs, at first, obliquely inwards, and then takes a straight course parallel with its fellow. The area thus defined is termed the areola; its breadth is equal to about one-third of the total transverse diameter of the carapace in this region.