Crayfish

Crayfish. The Crayfish is the most frequently quoted type of the Decapod Crustacea, as it is abundant in English streams, and its size is convenient for dissection. The name is a corruption of the French ecrevisse, and is not in any way connected with "fish." The Crayfish consists of three regions - head, thorax, and abdomen; each of these is composed of a series of segments, more or less united together, and each bearing a pair of jointed appendages. There are twenty segments in all - five in the head, eight in the thorax, and seven in the abdomen. The head and thorax are closely united, and the hard shell which protects the animal is there formed into a large plate, the cephalothorax; the division into head and thorax is, however, marked by a furrow. The head bears in front a sharp projecting point (the rostrum), on each side of which is a large compound eye borne on a short movable stalk; on the under surface the head bears five pairs of appendages, viz. the antennules, antennae, mandibles, and two maxillae; the two first are the feeling organs, while the three last surround the mouth and are concerned in the crushing of the food. The thorax bears three pairs of jaw feet or maxillipedes, the chelae or great claws, and four pairs of walking limbs. The abdomen is provided with six pairs of "swimmerets"; the last joint or telson has no appendages, but is flattened out with those of the preceding pair, and form the powerful swimming tail. The appendages, thus, have very varied functions, and their shape is consequently very different. Nevertheless, they are all constructed on one fundamental type. The simplest form is that of the swimmerets, which consists of a simple or two-jointed basal plate (the protopodite) bearing two filaments, of which the outer one is known as the exopodite, and the inner as the endopodite. The antennae and antennules consist of the same three parts: the other appendages are more complex; thus, in the third jaw foot or maxillipede the basal part consists of two joints, of which the upper one bears a gill and tuft of hair or setae; the endopodite consists of five joints, and the exopodite is slender and ends as a long filament. The mouth of the crayfish leads by a short oesophagus into a stomach, armed with a series of crushing teeth, forming a "digestive mill" ; from this a long straight intestine leads to the anus. The nerve system consists of a long chain of pairs of ganglia, united by a pair of nerves, cords, and transverse commissures. At the anterior end of the nervous system is the "brain," situated above the mouth, and giving off nerves to the eyes and feelers; a pair of "commissures" connect this to another nerve mass formed by the fusion of five pairs of ganglia; behind this is a chain of twelve pairs of ganglia, extending to the end of the abdomen. Respiration is effected by a series of nineteen pairs of gills, while there are also three rudimentary pairs; these are contained in a large branchial chamber on each side of the thorax; the gills are borne either on the limbs (podobranchs), on the side walls of the thorax (pleurobranchs), or on the membranes between the bases of the limbs and the walls (arthrobranchs). The water is driven through the branchial chamber by a spoon-shaped plate (scaphognathite) on the base of the second maxilla. The heart is placed on the middle of the dorsal side of the thorax, and is enclosed in a large "pericardium"; the blood is colourless. The kidneys are a pair of green glands, situated just in front of the mouth, and each opening on a small tubercle on the antenna. The auditory organs are a pair of small sacs, containing a series of hairs and grains (or otoliths), situated in the base of the antennules. The crayfish is carnivorous in habit; it usually walks about on the walking legs, but can swim back rapidly by the action of the abdomen. Its name is Astacus fluviatilis.