2. Each ganglion is divided into an anterior and posterior division, which are connected respectively with the motor and sensory nerves of the appendages. 3. The motor nerves of the appendages arise as well-defined axis-cylinder processes of nerve-cells, which are arranged in well-defined groups in the anterior division of the ganglion. 4. A separate innervation exists for the muscles and sensory surfaces of the trunk. The trunk-muscles consist of long bundles, from which slips pass off to the skin in each segment; they are thus imperfectly segmented. In accordance with this, a diffuse system of nerve-fibres passes to them from certain cells on the dorsal surface of each lateral half of the ganglion. These cell-groups are therefore very distinct from those which give origin to the motor {159}appendage-nerves, and, moreover, are not confined to the ganglion, but extend for some distance into the interganglionic region of the nerve-cords which connect together the ganglia of the ventral chain. Hardy's observations, therefore, combined with those of Milne-Edwards, lead to the conclusion that in such a primitive arthropod type as my theory postulates, each segment was supplied with separate sensory and motor somatic nerves, and with a pair of nerves of mixed function, devoted entirely to the innervation of the pair of appendages; that also, in the central nervous system, the motor nerve-centres were arranged in accordance with a double set of segmented muscles in two separate groups of nerve-cells. These nerve-cells in the one case were aggregated into well-defined groups, which formed the centres for the motor nerves of the markedly segmented muscles of the appendages, and in the other case formed a system of more diffused cells, less markedly aggregated into distinct groups, which formed the centres for the imperfectly segmented somatic muscles. Such an arrangement suggests that in the ancient arthropod type a double segmentation existed, viz. a segmentation of the body, and a segmentation due to the appendages. Undoubtedly, the segments originally corresponded absolutely as in Branchipus, and every appendage was attached to a well-defined separate body-segment. In, however, such an ancient type as Limulus, though the segmentation may be spoken of as twofold, yet the number of segments in the prosomatic and mesosomatic regions are much more clearly marked out by the appendages than by the divisions of the soma; for, in the prosomatic region such a fusion of somatic segments to form the tergal prosomatic carapace has taken place that the segments of which it is composed are visible only in the young condition, while in the mesosomatic region the separate somatic segments, though fused to form the mesosomatic carapace, are still indicated by the entapophysial indentations.