XII.

THE FRESH-WATER CRAYFISH (Astacus fluviatilis) AND THE LOBSTER (Homarus vulgaris).

THE Crayfish and the Lobster are inhabitants of the water, the former occurring in many of our rivers and the latter abounding on the rocky parts of the coasts of the European seas. They are bilaterally symmetrical animals, provided with many pairs of limbs, among which the large prehensile 'claws' are conspicuous. They are very active, walking and swimming with equal ease and sometimes propelling themselves backwards or forwards, with great swiftness, by strokes of the broad fin which terminates the body. They have conspicuous eyes, mounted upon moveable stalks, at the anterior end of the head; and two pairs of feelers, one pair of which are as long as the body, while the other pair are much shorter.

The body and limbs are invested by a strong jointed shell, or exoskeleton, which is a product of the subjacent epidermis, and consists of layers of membrane which remain soft and flexible in the interspaces between the segments of the body and limbs, but are rendered hard and dense elsewhere by the deposit of calcareous salts; the exoskeleton is deeply tinged with a colouring matter which turns red when exposed to the action of boiling water. The body presents an anterior division-the cephalothorax-covered

by a large continuous shield, or carapace; and a posterior division-the abdomen-divided into a series of segments which are moveable upon one another in the direction of the vertical median plane, so that the abdomen can be straightened out, in extension; or bent into a sharp curve, in flexion. Of these segments there are seven. The anterior six are the somites of the abdomen, and each of them has a pair of appendages attached to its ventral wall. The seventh bears no appendages and is termed the telson. The anus is situated on the ventral aspect, beneath the telson and behind the last somite.

A groove on the surface of the carapace, which is termed the cervical suture, separates an anterior division, which belongs to the head or cephalon, from a posterior division which covers the thorax; and the thoracic division of the carapace further presents a central region, which covers the head, and wide lateral prolongations, which pass downwards and cover the sides of the thorax, their free ventral edges being applied against the bases of the thoracic limbs. These are the branchiostegites. Each roofs over a wide chamber in which the gills are contained and which communicates with the exterior, below and behind, by the narrow interval between the edge of the branchiostegite and the limbs. Anteriorly and inferiorly, the branchial chamber is prolonged into a canal, which opens in front and below at the junction of the head with the thorax. In this canal there lies a flat oval plate-the scaphognathite-which is attached to the second pair of maxillæ and which plays a very important part in the performance of the function of respiration. Of the thoracic limbs themselves there are eight pairs, and, on the ventral face of the body, the lines of demarcation between the eight somites to which these limbs belong may be observed. There is no trace of any

corresponding divisions in the cephalothorax of the Lobster; but, in the Crayfish, the last thoracic somite is incompletely united with those which precede it. The four posterior pairs of thoracic limbs are those by which the animal walks and are termed the ambulatory legs. The next pair is formed by the great claws or chela. The anterior three pairs are bent up alongside the mouth and are moved to and from the median line so as to play the part of jaws, whence they are termed foot-jaws or maxillipedes. The external or third pair of these maxillipedes are much stouter and more like the ambulatory limbs than the rest, and the inner edges of their principal joints are toothed. The innermost or first pair of maxillipedes are broad, foliaceous and soft. When these foot-jaws are taken away, two pairs of soft foliaceous appendages come into view. They are attached to the hinder part of the cephalon and are the jaws or maxillæ. The second, or outermost, is produced, externally, into the scaphognathite, which will be seen to lie in a groove which separates the head from the thorax laterally and is the cervical groove.

Anterior to these maxillæ lie the two very stout mandibles. Between their inner toothed ends is the wide aperture of the mouth, bounded, in front, by a soft shield-shaped plate, the labrum; and behind, by another soft plate, divided by a deep median fissure into two lobes, which is the metastoma. Thus far, the surfaces of the somites to which the appendages are attached look downwards, when the body is straightened out and the carapace is directed upwards. But, in front of the mouth, the wall of the body to which the appendages are attached is bent up, at right angles to its former direction, and consequently looks forwards. This bend of the ventral wall of the body is the cephalic flexure. In correspondence with this change of position of the sur

face to which they are attached, the three pairs of appendages of the somites which lie in front of the mouth are directed either forwards, or forwards and upwards. The posterior pair consists of the long feelers or antenna: the next, of the short feelers or antennules; and the most anterior is formed by the short subcylindrical stalks (ophthalmites), on the ends of which the eyes are situated.

This enumeration shews that the Lobster and Crayfish have six pairs of abdominal appendages-the swimmerets; eight pairs of thoracic appendages (four pairs of ambulatory limbs, one pair of chelate prehensile limbs, three pairs of maxillipeds), and six pairs of cephalic appendages (two pairs of maxillæ, one pair of mandibles, one pair of antennæ, one pair of antennules, one pair of eyestalks), making in all twenty pairs of appendages. In correspondence with the number of appendages the body consists of twenty somites; of which six remain moveable upon one another to form the abdomen, while the other fourteen are united to form the cephalothorax.

The branchiostegite is an outgrowth of the dorsolateral region of the confluent thoracic somites. The serrated rostrum which ends the carapace is a fixed median prolongation of the dorsal wall of the anterior cephalic somites; while the telson is a moveable median prolongation of the dorsal wall of the sixth abdominal somite. The labrum and the metastoma are median growths of the sterna of the præoral and post-oral somites.

Thus the whole skeleton in these animals may be considered as a twentyfold repetition of the ring-like somite with its pair of appendages, which is seen in its simplest form in one of the abdominal somites. Moreover, notwithstanding the great variety of functions allotted to the various appendages, the study of the details of their structure (see Laboratory work) will shew that they are all re

ducible to modifications of a fundamental form, consisting of a basal joint (protopodite) with three terminal divisions (endopodite, exopodite, epipodite).

As has been already said, the Lobster and Crayfish are bilaterally symmetrical; that is to say, a median vertical plane passing through the mouth and anus divides them into two similar halves. This symmetry is exhibited not merely by the exterior of the body and the correspondence of the paired limbs, but extends to the internal organs; the alimentary canal and its appendages, the heart, the nervous system, the muscles and the reproductive organs, being disposed so as to be symmetrical in relation to the median vertical plane of the body.

The wide gullet leads almost vertically into the spacious stomach, and both are lined by a chitinous continuation of the exoskeleton. The stomach is divided by a transverse constriction into a spacious cardiac, and a much smaller pyloric division, from which latter the intestine passes. The walls of the anterior half of the cardiac sac are thin and membranous, but, in the posterior half, they become calcified so as to give rise to a gastric skeleton of considerable complexity. The chief part of this skeleton consists of a median dorsal T-shaped cardiac' ossicle, the cross-piece of which forms a transverse arch, while its long median process extends backwards in the middle line. The ends of the transverse arch are articulated obliquely with two small 'antero-lateral' pieces, the extremities of which again are articulated with postero-lateral pieces, and these unite with a cross-piece, the 'pyloric' ossicle, which arches over the roof of the pyloric division of the stomach. In this manner a sort of hexagonal frame with moveable joints is formed, and the median process projects backwards so far, as to end