The Crayfish: An Introduction to the Study of Zoology.

by Thomas Henry Huxley

If a piece of the crayfish’s skeleton is placed in strong vinegar, abundant bubbles of carbonic acid gas are given off from it, and it rapidly becomes converted into a soft laminated membrane, while the solution will be found to contain lime. In fact the exoskeleton is composed of a peculiar animal matter, so much impregnated with carbonate and phosphate of lime that it becomes dense and hard.

FIG. 2.—Astacus fluviatilis.—Dorsal or tergal views (nat. size). A, male; B, female:—bcg, branchio-cardiac groove, which marks the boundary between the pericardial and the branchial cavities; cg, cervical groove; these letters are placed on the carapace; r, rostrum; t, t′, the two divisions of the telson; 1, eye-stalks; 2, antennules; 3, antennæ; 20, lateral lobes of tail-fin; XV–XX, somites of the abdomen.

It will be observed that the body of the crayfish is naturally marked out into several distinct regions. There {19} is a firm and solid front part, covered by a large continuous shield, which is called the carapace; and a jointed hind part, commonly termed the tail (fig. [2]). From the perception of a partially real, and partially fanciful, analogy with the regions into which the body is divided in the higher animals, the fore part is termed the cephalo-thorax, or head (cephalon) and chest (thorax) combined, while the hinder part receives the name of abdomen.

Now the exoskeleton is not of the same constitution throughout these regions. The abdomen, for example, is composed of six complete hard rings (fig. [2], XV–XX), and a terminal flap, on the under side of which the vent (fig. [3], a) is situated, and which is called the telson (fig. [2], t, t′). All these are freely moveable upon one another, inasmuch as the exoskeleton which connects them is not calcified, but is, for the most part, soft and flexible, like the hard exoskeleton when the lime salts have been removed by acid. The mechanism of the joints will have to be attentively considered by-and-by; it is sufficient, at present, to remark that, wherever a joint, exists, it is produced in the same fashion, by the exoskeleton remaining soft in certain regions of the jointed part.

The carapace is not jointed; but a transverse groove is observed about the middle of it, the ends of which run down on the sides and then turn forwards (figs. [1] and [2], cg). This is called the cervical groove, and it marks off {20} the region of the head, in front, from that of the thorax behind.

The thorax seems at first not to be jointed at all; but if its under, or what is better called its sternal, surface is examined carefully, it will be found to be divided into as many transverse bands, or segments, as there are pairs of legs (fig. [3]); and, moreover, the hindermost of these segments is not firmly united with the rest, but can be moved backwards and forwards through a small space (fig. [3], B; xiv).

Attached to the sternal side of every ring of the abdomen of the female there is a pair of limbs, called swimmerets. In the five anterior rings, these are small and slender (fig. [3], B; 15, 19); but those of the sixth ring are very large, and each ends in two broad plates (20). These two plates on each side, with the telson in the middle, constitute the flapper of the crayfish, by the aid of which it executes its retrograde swimming movements. The small swimmerets move together with a regular swing, like paddles, and probably aid in propelling the animal forwards. In the breeding female (B), the eggs are attached to them; while, in the male, the two anterior pairs (A; 15, 16) are converted into the peculiar styles which distinguish that sex.

FIG. 3.—Astacus fluviatilis.—Ventral or sternal views (nat. size). A, male; B, female:—a, vent; gg, opening of the green gland; lb, labrum; mt, metastoma or lower lip; od, opening of the oviduct; vd, that of the vas deferens. 1, eye-stalk; 2, antennule; 3, antenna; 4, mandible; 8, second maxillipede; 9, third or external maxillipede; 10, forceps; 11, first leg; 14, fourth leg; 15, 16, 19, 20, first, second, fifth, and sixth abdominal appendages; X., XI., XIV., sterna of the fourth, fifth, and eighth thoracic somite; XVI., sternum of the second abdominal somite. In the male, the 9th to the 14th and the 16th to the 19th appendages are removed on the animal’s left side: in the female, the antenna (with the exception of its basal joint) and the 5th to the 14th appendages on the animal’s right are removed; the eggs also are shown attached to the swimmerets of the left side of the body.