The =nervous system= of _Branchipus_ may be described as an illustration of the condition prevailing in the group. The brain consists of two closely united ganglia, in each of which three main regions may be distinguished; a ventral anterior lobe, a dorsal anterior lobe, and a posterior lobe. The ventral anterior lobes give off nerves to the median eye, to the dorsal organ, and to a pair of curious sense-organs, comparable with the larval sense-knobs of many higher forms, situated one on each side of the median eye; in late larvae Claus describes the terminal apparatus of each frontal sense-organ as a single large hypodermic cell; W. K. Spencer[20] has lately described several terminal cells, containing peculiar chitinous bodies, in the adult. The homologous sense-organs of _Limnetis_ are apparently olfactory. The dorsal anterior lobes give off the large nerves to the lateral eyes, while the posterior lobes supply the first antennae. The oesophageal connectives have a coating of ganglion-cells, and some of these form the ganglion of the second antenna, the nerve to this appendage leaving the connective just behind the brain. The post-oral nerve-cords are widely separate, each of them dilating into a ganglion opposite every appendage, the two ganglia being connected by two transverse commissures. The ganglia of the three cephalic jaws, so often fused in the higher Crustacea, are here perfectly distinct. Closely connected with each thoracic ganglion is a remarkable unicellular gland, opening to the exterior near the middle ventral line; it is conceivable that these cells may be properly compared with the larval nephridia of a Chaetopod,[21] but no evidence in support of such a comparison has yet been adduced. Behind the genital segments, where there are no limbs, the nerve-cords run backwards without dilating into segmental ganglia, except in the anterior two abdominal segments where small ganglionic enlargements occur. In Apodidae, on the other hand, those segments which carry more than one pair of appendages have as many pairs of ganglia, united by transverse commissures, as they have limbs. A stomatogastric nervous system exists in _Apus_, where a nerve arises on each side from the first post-oral commissure, and runs forward to join its fellow of the opposite side on the anterior wall of the oesophagus. From the loop so formed a larger median and a series of smaller lateral nerves pass to the wall of the alimentary canal. A second nerve to the oesophagus is given off from the mandibular ganglion of each side.