The Crayfish: An Introduction to the Study of Zoology.

by Thomas Henry Huxley

There are two small pointed teeth, one under each of the lateral teeth, and each of these is supported by {56} a broad plate, hairy on its inner surface, which enters into the lateral wall of the cardiac chamber. There are various other smaller skeletal parts, but the most important are those which have been described; and these, from what has been said, will be seen to form a sort of hexagonal frame, with more or less flexible joints at the angles, and having the anterior and the posterior sides {57} connected by a bent jointed middle bar. As all these parts are merely modifications of the hard skeleton, the apparatus is devoid of any power of moving itself. It is set in motion, however, by the same substance as that which gives rise to all the other bodily movements of the crayfish, namely, muscle. The chief muscles which move it are four very strong bundles of fibres. Two of these are attached to the front crosspiece, and proceed thence, upwards and forwards, to be fixed to the inner face of the carapace in the front part of the head (figs. [5], [6], and [12], ag). The two others, which are fixed into the hinder crosspiece and hinder lateral pieces, pass upwards and backwards, to be attached to the inner face of the carapace in the back part of the head (pg). When these muscles shorten, or contract, they pull the front and back crosspieces further away from one another; consequently, the angle between the handles becomes more open and the tooth which is borne on their ends travels downwards and forwards. But, at the same time, the angle between the side bars becomes more open and the lateral tooth of each side moves inwards till it crosses in front of the middle tooth, and strikes against this and the opposite lateral tooth, which has undergone a corresponding change of place. The muscles being now relaxed, the elasticity of the joints suffices to bring the whole apparatus back to its first position, when a new contraction brings about a new clashing of the teeth. Thus, by the alternate contraction and relaxation of these two pair of muscles, the {58} three teeth are made to stir up and crush whatever is contained in the cardiac chamber. When the stomach is removed and the front part of the cardiac chamber is cut away, the front cross-piece may be seized with one pair of forceps and the hind cross-piece with another. On slightly pulling the two, so as to imitate the action of the muscles, the three teeth will be found to come together sharply, exactly in the manner described.

Works on mechanics are full of contrivances for the conversion of motion; but it would, perhaps, be difficult to discover among these a prettier solution of the problem; given a straight pull, how to convert it into three simultaneous convergent movements of as many points.

What I have called the filter is constructed mainly out of the chitinous lining of the pyloric chamber. The aperture of communication between this and the cardiac chamber, already narrow, on account of the constriction of the walls of the stomach at this point, is bounded at the sides by two folds; while, from below, a conical tongue-shaped process (figs. [6], [10], and [11], cpv), the surface of which is covered with hairs, further obstructs the opening. In the posterior half of the pyloric chamber, its side walls are, as it were, pushed in; and, above, they so nearly meet in the middle line, that a mere vertical chink is left between them; while even this is crossed by hairs set upon the two surfaces. In its lower half, however, each side wall curves outwards, and forms a cushion-shaped surface (fig. [10], cs) which looks downwards and inwards. If the {59} floor of the pyloric chamber were flat, a wide triangular passage would thus be left open in its lower half. But, in fact, the floor rises into a ridge in the middle, while, at the sides, it adapts itself to the shape of the two cushion-shaped surfaces; the result of which is that the whole cavity of the posterior part of the pyloric division of the stomach is reduced to a narrow three-rayed fissure. In transverse section, the vertical ray of this fissure is straight, while the two lateral ones are concave upwards (fig. [9], E). The cushions of the side walls are covered with short close-set hairs. The corresponding surfaces of the floor are raised into longitudinal parallel ridges, the edge of each of which is fringed with very fine hairs. As everything which passes from the cardiac sac to the intestine must traverse this singular apparatus, only the most finely divided solid matters can escape stoppage, so long as its walls are kept together.

Finally, at the opening of the pyloric sac into the intestine, the chitinous investment terminates in five symmetrically arranged processes, the disposition of which is such that they must play the part of valves in preventing any sudden return of the contents of the intestine to the stomach, while they readily allow of a passage the other way. One of these valvular processes is placed in the middle line above (figs. [10] and [11], v¹). It is longer than the others and concave below. The lateral processes (v²,) of which there are two on each side, are triangular and flat. {60}

FIG. 11.—Astacus fluviatilis.—View of the roof of the stomach, the ventral wall of which, and of the mid-gut, is laid open by a longitudinal incision (× 4). On the right side (the left in the figure), the lateral tooth is cut away, as well as the floor of the lateral pouch. The letters have the same signification as in fig. [10].

The cuticular lining which gives rise to all the complicated apparatus which has just been described, must not be confounded with the proper wall of the stomach, which invests it, and to which it owes it origin, just as the cuticle of the integument is produced by the soft {61} true skin which lies beneath it. The wall of the stomach is a soft pale membrane containing variously disposed muscular fibres; and, beyond the pylorus, it is continued into the wall of the intestine.

It has already been mentioned that the intestine is a slender and thin-walled tube, which passes straight through the body almost without change, except that it becomes a little wider and thicker-walled near the vent. Immediately behind the pyloric valves, its surface is quite smooth and soft (figs. [9], [10], and [12], mg), and its floor presents a relatively large aperture, the termination of the bile duct (fig. [12], bd, fig. [10], hp), on each side. The roof is, as it were, pushed out into a short median pouch or cæcum (cæ). Behind this, its character suddenly changes, and six squarish elevations, covered with a chitinous cuticle, encircle the cavity of the intestine (r). From each of these, a longitudinal ridge, corresponding with a fold of the wall of the intestine, takes its rise, and passes, with a slight spiral twist, to its extremity (hg). Each of these ridges is beset with small papillæ, and the chitinous lining is continued over the whole to the vent, where it passes into the general cuticle of the integument, just as the lining of the stomach is continuous with the cuticle of the integument at the mouth. The alimentary canal may, therefore, be distinguished into a fore and a hind-gut (hg), which have a thick internal lining of cuticular membrane; and a very short mid-gut (mg), which has no thick cuticular layer. It will be of {63} importance to recollect this distinction by-and-by, when the development of the alimentary canal is considered.

FIG. 12.—Astacus fluviatilis.—A dissection of a male specimen from the right side (nat. size). a, anus; aa, antennary artery, cut short; ag, anterior gastric muscles, the right cut away to its insertion; bd, aperture of right bile duct; cm, constrictor muscles of stomach; cæ, cæcum; cpm, right cardio-pyloric muscle; cs, cardiac portion of stomach; cm, extensor muscles of abdomen; fm, flexor muscles of abdomen; ga, gastric artery; gn. 1, supraœsophageal ganglion; gn. 2, sub-œsophageal ganglion; gn. 13, last abdominal ganglion; h, heart; ha, hepatic artery; hg, hind-gut; iaa, inferior abdominal artery; la, right lateral aperture of heart; lr, left liver; mg, mid-gut; oa, ophthalmic artery; œ, œsophagus; pg, posterior gastric muscles, the right cut away to its insertion; ps, pyloric portion of stomach; sa, sternal artery; saa, superior abdominal artery; t (to the left), telson; t (near the heart), testis; vd, left vas deferens; vd′, aperture of left vas deferens; 2, right antennule; 4, left mandible; 9, left external maxillipede; 10, left forceps; 15, first, 16, second, and 20, sixth abdominal appendages of the left side.