The English and Irish Crayfish is widely spread over the Continent. It constitutes the variety of Astacus fluviatilis known as A. torrentium, the Ecrevisse a pieds blancs. There is a Crayfish which is found also widely spread over the Continent and which resembles A. torrentium very closely. This constitutes the variety of A. fluviatilis known as A. nobilis, the Ecrevisse a pieds rouges.
The body walls or integument consist of (1)cuticular structures for the most part calcified; of (2) a single layer of ectoderm or hypoderm cells (=chitinogenous cells); and of (3) connective tissue imbedding pigment cells, bloodvessels and nerves.
The cuticular structures differ somewhat in the hard calcified and the soft intersegmental regions. The former is composed of four distinct strata: (1) a cuticle, (2) a pigmented calcified layer, (3) a non-pigmented thick layer containing a very large proportion of calcareous matter, and (4) a non-calcified softish and thin layer. The organic substratum is chitinoid. The salts are Calcium carbonate and phosphate, chiefly the former.
(1) The cuticle is structureless, resistent, and of a yellowish tint. It shows areae corresponding to the outlines of the ectoderm cells, and inside these areae slight ridges. The latter are supposed by Braun to be due to the coalescence with the young cuticle during its formation of processes similar to those of the intestinal cuticle. Both areae and ridges are more distinct in the newly formed cuticle.
(2) The pigmented layer consists of a system of fine lamellae, parallel to the surface, and alternately more and less refractile, the latter being mere lines. The lamellae are perforated by minute close-set vertical pores. The pigment lies between the more refractile lamellae in the form of bluish-black granules, sometimes furnished with processes.
(3) The third layer makes up the chief substance of the hard parts. Its refractile lamellae are thick, well calcified, and perforated by pores continuous with those of the pigmented layer. The lamellae become thinner towards the inner surface, where they merge insensibly into the fourth layer.
(4) This layer is very distinct in the Lobster. It is not recognised by Braun, but according to Vitzou it is composed of delicate non-calcified lamellae, and the innermost show impressions or outlines corresponding to the ectoderm cells. It is said to possess but few vertical pores.
There are certain variations in different regions from this typical structure The knobs on the opposing surfaces of the chelae, certain spines, e. g. those on the posterior dorsal edge of the basal joint of the exopodite in the last pair of swimmerets, are colourless. The pigmented layer is absent, and the cuticle is greatly thickened in these places. In the articular and intersegmental membranes the cuticle is present, but the remaining layers are represented by non-calcified lamellae not distinguishable into systems. The inner wall of the branchiostegite is extremely thin, and is composed of a membrane resembling the cuticle. Areae are absent here as they are on the eye-stalk, and in the oesophagus and stomach. They are very well marked in the intestine, and each area supports 3-6 pointed ridges. The cuticular structures of the alimentary canal are composed of a cuticle and underlying lamellae, which are thickened and calcified only in the gastric ossicles. Vertical pores are found also only in the same structures. There are integumental setae or hairs and glands. The setae are either hollow or solid. The hooked setae on the inner wall of the branchiostegite are solid, and certain solid setae in the oesophagus and stomach are probably simply processes of cuticle.
Hollow setae are either closed or open at the base. The former are most numerous, and their stem is beset with solid barbs, i. e. they are feathered. The latter are chiefly confined to the antennae. They are short, thick-walled, and their stems are destitute of barbs. A pore-canal pierces all the layers of the integument except the cuticle, and leads to the base of each seta. The cuticle is continuous with the base of the hair, which is generally lodged in a slight depression. The pore-canal contains a process of the ectoderm, in which nuclei are present. It is prolonged internally below the integument by the hair-tube. This structure has nucleated walls, and expands at its inner extremity into a bulb in which cell-outlines are distinguishable. It is about half the length of a hair, and contains a hair papilla which fills its cavity. A formation of new hairs precedes the formation of a new cuticle etc The barbs, when present, are first formed, then the walls of the stem both alike from the papilla and the walls of the tube. The tip of the hair is slightly hooked, and fits into the pore-canal of its predecessor. When the old integument is cast off the new hairs are therefore evaginated mechanically.
The soft substance contained in the stem atrophies and a new hair-papilla is formed, as is believed by Braun from the walls of the tube.
The glands of the integument are (1) the glands of the roof of the branchial cavity, and (2) the cement glands found only in the female. The former were discovered by Leydig. They lie in the substance of the branchiostegite and open singly on the roof of the branchial cavity. They are tubular in structure and are but slightly lobed. The gland cells are columnar and pointed. The cement glands were discovered and investigated by Braun, and have been found by him in other Decapoda. In the Crayfish they extend over the anterior two-thirds of the ventral surfaces of the abdominal pleura, and thence they spread along the edges of the sterna almost continuously. They also cover about a third of the base of the exo- and endo-podite of the last pair of swimmerets. These cement glands undergo a periodical development, 5-8 weeks before the eggs are laid in November or December, and then they give the parts a whitish appearance. The glands'themselves are tubular with rounded or polyhedric cells supported by a basement membrane: their ducts rarely open singly, but as a rule in groups. Lereboullet, who first observed the white appearance of the abdomen, but who failed to recognise the glands, states that their secretion coagulates on exposure to water.