Dissected so as to show its nervous system.

The supra-oesophageal ganglion and the twelve post-oral ganglia of the adult Crayfish, of which six belong to the thorax and six to the abdomen, have been displayed by the removal of the whole tergal region of the body, of the viscera of organic life, and the endophragmal skeleton in the thorax. The oesophagus, through which a black bristle has been passed, and a small terminal portion of the intestine remain in situ.

The supra-oesophageal ganglion was seen by Rathke to be made up of two rudiments in the embryo; of which the posterior, or the one placed nearest to the mouth, was the larger and supplied the first and second antennae. The ganglion itself in the adult gives off nerves to the eye, the eye-muscles, to the integument of the head, and the first and second antennae, besides furnishing two azygos nerves, one anterior, the other posterior, to the stomato-gastric nerve. The superior and inferior roots of this nerve (infra), together with the two commissures to the sub-oesophageal or first post-oral ganglion, are seen passing over a piece of blue paper placed under them in front of the oesophagus. The left end of this blue paper rests on the secreting portion of the left antennary or green gland. The infra-oesophageal ganglion is the largest of the post-oral series and innervates no less than six pairs of appendages, viz. the mandibles, the two pairs of maxillae, and the three pairs of maxillipeds or foot-jaws. In the developing Crayfish, as shown by Rathke, this mass is represented by six pairs of white specks. It is followed by five thoracic ganglia, which remain distinct and correspond in the adult as well as in the embryo of Macrurous Decapods to the five pairs of thoracic feet.

Each ganglion is connected to its successor by two longitudinal commissures, showing the primitive bilateral composition of the chain. The commissures between the third and fourth ganglia are widely separate for the passage of the sternal artery seen in Preparation 33. The fourth and fifth ganglia are approximated. The first abdominal ganglion is some distance behind the last thoracic, and all the six abdominal ganglia are equidistant one from the other. A slip of blue paper has been placed under the third and fourth, and another under the commissure to the last of the series. The commissural cords are clearly double. The third ganglion is seen to give off a pair of nerves on each side, while another pair springs from the commissures immediately behind the ganglion. The anterior nerve on each side goes to the swimmerets; the posterior and the commissural pair to the muscles of the same, i. e. the third somite. All the abdominal ganglia resemble the third pair in these points with the exception of the terminal ganglion, which may be seen to give off a large number of nerves. Accurate investigations have shown that of these there are five pairs and one posterior, median and azygos nerve.

This azygos nerve supplies the termination of the intestine; and the nerves to either side of it, i. e. the fifth or innermost pair, are destined for the telson. The two outermost pairs of nerves, i. e. the first and second, go to the exopodite, and the third pair to the endopodite of the last enlarged pair of swimmerets. The fourth pair, according to Krieger, supplies muscles in the same manner as do the commissural pairs of nerves corresponding to the five foregoing ganglia.

The infra-oesophageal and the thoracic ganglia lie in the sternal canal formed by processes of the apodemata. The roof of this canal has been cut away to expose the nerve chain, but parts of the apodemata may be seen on either side of it in the shape of vertical tubular processes.

The two eyes with their stalks, the bases of the first and second antennae, are shown by the removal of the overhanging rostrum. The surface of the basal joint of each first antenna thus exposed is the one that contains the aperture into the auditory sac. The aperture itself is concealed by setae.

The summits of the branchial plumes are well seen in this specimen between the branchiostegite and the epimera of the thoracic somites. From the internal aspect of the epimera, the muscles which move the limbs upon the thorax are seen trending downwards and bifurcating as they pass between the sections of the endophragmal skeleton to their insertions.

The sub-oesophageal ganglion gives off six inferior and four superior or dorsal nerves. The six inferior are destined for the mouth-parts. The mandibular nerve accompanies the commissures round the oesophagus for a certain distance. The last nerve, which goes to the third maxilliped, arises at some distance behind that for the second maxilliped, and the part of the ganglion from which it springs has a certain amount of distinctness or individuality. Of the four superior nerves the first is of considerable size and innervates the scapho-gnathite. The three remaining nerves are fine, and their destination unknown.

Each of the five thoracic ganglia gives off two pairs of nerves: an anterior large pair destined for the limb and the gills belonging to the somite, and a posterior fine pair destined for the corresponding thoracic muscles.

The median azygos nerve given off by the last abdominal ganglion divides, according to Lemoine, into two branches, a posterior anal branch and an anterior intestinal branch. The latter subdivides into (1) a branch to the anal end of the intestine; (2) a branch which courses along the ventral surface of the intestine, to which it gives twigs from spot to spot; and (3) a branch which turns round* the intestine and runs upon its dorsal aspect. Lemoine traced these two last-mentioned branches as far forwards as the genitalia.

The nerve-factors which make up the stomatogastric system are derived from two sources, from the supra-oesophageal ganglion itself, and from the commissures connecting it to the infra-oesophageal ganglion at the spot where these commissures come into contact with the walls of the oesophagus. The nerves derived from the first-named source are two, a superior azygos nerve and an inferior azygos nerve. The former has a small ganglion close to its origin; it is short and runs upwards, i.e. dorsally. The latter is long and runs backwards and downwards. Two small ganglia, the mandibular or oesophageal ganglia, recently investigated by Krieger, give origin to the second set of factors named above. They are semi-oval and lie on the ventral side of the commissures, and from each of them spring at least three nerves. One is external and bends down, branching on the oesophagus; the two others, the superior and inferior roots, are internal and pass forwards between the commissures. The inferior roots unite together to form a single trunk, to which the superior roots then unite.

The single nerve thus formed is joined by the inferior azygos nerve from the supra-oesophageal ganglion, and constitutes a median nerve which runs upwards in front of the stomach, giving off one after another three branches to that organ (Lemoine). It then unites with the superior azygos nerve 1 from the supra-oesophageal ganglion. The single trunk formed by this union bends round the stomach on to its dorsal aspect. Close to the point of union it gives a nerve to the stomach and twigs to the anterior gastric muscles through which it passes. It then forms the stomatogastric ganglion, from which spring two nerves, an upper, the cardiac nerve of Lemoine, lodged in the integument and going to the heart, and an inferior or gastro-hepatic which lies on the dorsal wall of the stomach. The latter passes backwards and divides posteriorly into the terminal branches. Between these and the stomatogastric ganglion is a slight swelling from which rise the two lateral branches. Both lateral and terminal branches pass downwards. The latter supply the posterior gastric muscles.

They eventually distribute themselves to the liver so-called and various stomachal muscles, and anastomose both with the lateral branches and theposterolateralnerve which arises from each oesophageal commissure dorsally to the oesophageal ganglion, and passes upwards on the oesophagus. These various nerves give off numerous branches which have recently been investigated in detail by Mocquard in various Decapoda.

The ganglia consist of central masses of Leydig's 'punkt-substanz' formed by dense networks of fine nerve-fibrils, and external masses of ganglion cells, varying in size. The ganglion cells themselves differ in the same respect: the smallest possess but little protoplasm. Each cell is contained in a connective tissue capsule. Their processes, though numerous, originate in most instances from one surface or pole. The nerves are tubular, and, according to Krieger, consist of an external sheath and homogeneous fluid contents, but Freud states that there are delicate fibrillae imbedded in this homogeneous substance; and he traces the same distinction, viz. a homogeneous matrix and imbedded fibrillar network, in the bodies of the ganglion cells. The nerves branch repeatedly. There is a tough elastic perineurium or common investment, composed of decussating fibres and covered within and without by cellular connective tissue.

Histology of nerve-cord, nerves, etc. In Astacus, Krieger, Z. W. Z. xxxiii. 1880;

1 This root is not mentioned by Mocquard, but is figured by both Krieger and Lemoine, and I have found it myself more than once.

Freud, SB. Akad. Wien, lxxxv. Abth. 3, 1882. In Decapoda, Yung, A. Z. Expt. vii. 1878.

Stomatogastric system with figures, and posterior intestinal nerves. Lemoine, A. Sc. N. (5), ix. 1868; Mocquard, A. Sc. N. (6), xvi. 1883.