The Crayfish: An Introduction to the Study of Zoology.

by Thomas Henry Huxley

And here we must trench somewhat upon the province of Morphology, as some of these pieces of apparatus are complicated; and their action cannot be comprehended without a certain knowledge of their anatomy.

The mouth of the crayfish is a longitudinally elongated, parallel-sided opening, in the integument of the ventral or sternal aspect of the head. Just outside its lateral boundaries, the strong mandibles project, one on each side (fig 3, B; 4); their broad crushing surfaces, which are turned towards one another, are therefore completely external to the oral cavity. In front, the mouth is overlapped by a wide shield-shaped plate termed the upper lip, or labrum (figs. [3] and [6], lb); while, immediately behind the mandibles, there is, on each side, an elongated fleshy lobe, joined with its fellow by the posterior boundary of the mouth. These together constitute the metastoma (fig. [3], B; mt), which is sometimes called the lower lip. A short wide gullet, termed the œsophagus (fig. [6], oe), leads directly upwards into a spacious bag, the stomach, which occupies almost the whole cavity of the head. It is divided by a constriction into a large anterior chamber (cs), into the under face of which the {52} gullet opens, and a small posterior chamber (ps), from which the intestine (hg) proceeds.

In a man’s stomach, the opening by which the gullet communicates with the stomach is called the cardia, while that which places the stomach in communication with the intestine is named the pylorus; and these terms having been transferred from human anatomy to that of the lower animals, the larger moiety of the crayfish’s stomach is called the cardiac division, while the smaller is termed the pyloric division of the organ. It must be recollected, however, that, in the crayfish, the so-called cardiac division is that which is actually furthest from the heart, not that which is nearest to it, as in man.

The gullet is lined by a firm coat which resembles thin parchment. At the margins of the mouth, this strong lining is easily seen to be continuous with the cuticular exoskeleton; while, at the cardiac orifice, it spreads out and forms the inner or cuticular wall of the whole gastric cavity, as far as the pylorus, where it ends in certain valvular projections. The chitinous cuticle which forms the outermost layer of the integument is thus, as it were, turned in, to constitute the innermost layer of the walls of the stomach; and it confers upon them so great an amount of stiffness that they do not collapse when the organ is removed from the body. Furthermore, just as the cuticle of the integument is calcified to form the hard parts of the exoskeleton, so is the cuticle of the stomach calcified, or otherwise hardened, to give rise, in the first {53} place, to the very remarkable and complicated apparatus which has already been spoken of, as a sort of gastric mill or food-crusher; and, secondly, to a filter or strainer, whereby the nutritive juices are separated from the innutritious hard parts of the food and passed on into the intestine.

FIG. 9.—Astacus fluviatilis.—A, the stomach with its outer coat removed, seen from the left side; B, the same viewed from the front, after removal of the anterior wall; C, the ossicles of the gastric mill separated from one another; D, the prepyloric ossicle and median tooth, seen from the right side; E, transverse section of the pyloric region along the line xy in A (all × 2). c, cardiac ossicle; cpv, cardio-pyloric valve; lp, lateral pouch; lt, lateral tooth, seen through the wall of the stomach in A; mg, mid-gut; mt, median tooth, seen through the wall of the stomach in A; œs, œsophagus; p, pyloric ossicle; pc, pterocardiac ossicle; pp, prepyloric ossicle; uc, uro-cardiac process; t, convexities on the free surface of its hinder end; v¹, median pyloric valve; zc, zygocardiac ossicle.

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The gastric mill begins in the hinder half of the cardiac division. Here, on the upper wall of the stomach, we see a broad transverse calcified bar (figs. [9]–11, c) from the middle of the hinder part of which another bar (uc), united to the first by a flexible portion, is continued backwards in the middle line. The whole has, therefore, somewhat the shape of a cross-bow. Behind the first-mentioned piece, the dorsal wall of the stomach is folded in, in such a manner as to give rise to a kind of pouch; and the second piece, or what we may call the handle of the crossbow, lies in the front wall of this pouch. The end of this piece is dense and hard, and its free surface, which looks into the top of the cardiac chamber, is raised into two oval, flattened convex surfaces (t). Connected by a transverse joint with the end of the handle of the crossbow, there is another solid bar, which ascends obliquely forwards in the back wall of the pouch (pp). The end which is articulated with the handle of the crossbow is produced into a strong reddish conical tooth (mt), curved forwards and bifurcated at the summit; consequently, when the cavity of the stomach is inspected from the fore part of the cardiac pouch (fig. [9], B), the two-pointed curved tooth (mt) is seen projecting behind the convex surfaces (t), in the middle line, into the interior of that cavity. The joint which connects the handle of the crossbow with the hinder middle piece is elastic; hence, if the two are straightened out, they return to their bent disposition as soon as they are released. The upper end of {55} the hinder middle piece (pp) is connected with a second flat transverse plate which lies in the dorsal wall of the pyloric chamber (p). The whole arrangement, thus far, may be therefore compared to a large cross-bow and a small one, with the ends of their handles fastened together by a spring joint, in such a manner that the handle of the one makes an acute angle with the handle of the other; while the middle of each bow is united with the middle of the other by the bent arm formed by the two handles. But, in addition to this, the outer ends of the two bows are also connected together. A small, curved, calcified bar (pc) passes from the outer end of the front crosspiece downwards and outwards in the wall of the stomach, and its hinder and lower extremity is articulated with another larger bar (zc) which runs upwards and backwards to the hinder or pyloric crosspiece, with which it articulates. Internally, this piece projects into the cardiac cavity of the stomach as a stout elongated reddish elevation (lt), the surface of which is produced into a row of strong sharp, transverse ridges, which diminish in size from before backwards, and constitute a crushing surface almost like that of the grinder of an elephant. Thus, when the front part of the cardiac cavity is cut away, not only are the median teeth already mentioned seen, but, on each side of them, there is one of these long lateral teeth.

FIG. 10.—Astacus fluviatilis.—Longitudinal section of the stomach (× 4), c, cardiac ossicle; cæ, cæcum; c.p.v, cardio-pyloric valve; cs, cushion-shaped surface; hg, hind-gut; hp, aperture of right bile duct; lp, lateral pouch; lt, lateral teeth; mg, mid-gut; mt, median tooth; œs, œsophagus; p, pyloric ossicle; pc, pterocardiac ossicles; pp, prepyloric ossicle; uc, urocardiac process; v¹, median pyloric valve; v², lateral pyloric valve; x, position of gastrolith; zc, zygocardiac ossicle.