[Illustration: FIG. 25.—_Astacus fluviatilis._—The central nervous system seen from above (nat. size). _a_, vent; _an_, antennary nerve; _a′n_, antennulary nerve; _c_, circumœsophageal commissures; _gn._ 1, supraœsophageal ganglion; _gn._ 2, infraœsophageal ganglion; _gn._ 6, fifth thoracic ganglion; _gn._ 7, last thoracic ganglion; _gn._ 13, last abdominal ganglion; _œs_, œsophagus in cross section; _on_, optic nerve; _sa_, sternal artery in cross section; _sgn_, stomatogastric nerve.] The chief ganglia of the crayfish are disposed in a longitudinal series in the middle line of the ventral aspect of the body close to the integument (fig. 25). In the abdomen, for example, six ganglionic masses are readily observed, one lying over the sternum of each somite, connected by longitudinal bands of nerve fibres, and giving off branches to the muscles. On careful examination, the longitudinal connecting bands, or _commissures_ (fig. 24, _co_), are seen to be double, and each mass appears slightly bilobed. In the thorax, there are six, larger, double ganglionic masses, likewise connected by double commissures; and the most anterior of these, which is the largest (fig. 25, _gn._ 2), is marked at the sides by notches, as if it were made up of several pairs of ganglia, run together into one continuous whole. In front of this, two commissures (_c_) pass forwards, separating widely, to give room for the gullet (_œs_), which passes between them; while in front of the gullet, just behind the eyes, they unite with a transversely elongated mass of ganglionic substance (_gn._ 1), termed the _brain_, or _cerebral ganglion_. All the motor nerves, as has been said, are traceable, directly or indirectly, to one or other of these thirteen sets of ganglia; but other nerves are given off from the ganglia, which cannot be followed into any muscle. In {106} fact, these nerves go either to the integument or to the organs of sense, and they are termed _sensory nerves_.