The segmented annelid type of nervous system consists of a supra-oesophageal ganglion, composed of the fused ganglia belonging to the pre-oral segments, and an infra-oesophageal chain of separate ganglia. With the concentration and modification around the mouth of the most anterior locomotor appendages to form organs for prehension and mastication of food, a corresponding concentration and fusion of the ganglia belonging to these segments takes place, so that finally, in the higher annelids, and in most of the great arthropod group, a fusion of a number of the most anterior ganglia has taken place to form the infra-oesophageal ganglion-mass. The infra-oesophageal ganglia which are the first to fuse are those which supply the most anterior portion of the animal with nerves, and include always those anterior appendages which are modified for mastication purposes. To this part the name _prosoma_ has been given; in many cases it forms a well-defined, distinct portion of the animal. Succeeding this prosoma or masticatory region, there occurs in all gill-bearing arthropods a respiratory region, in many cases more or less distinctly defined, which has received the name of _mesosoma._ The rest of the body is called the _metasoma_. In accordance with this nomenclature the central nervous system of many of the Arthropoda may be divided as follows:-- 1. Pre-oral, or supra-oesophageal ganglia. 2. Infra-oral, or infra-oesophageal ganglia and ventral chain, which consist of three groups: prosomatic, mesosomatic, and metasomatic ganglia. The infra-oesophageal ganglion-mass, then, in most of the Arthropoda may be spoken of as formed by the fusion of the prosomatic or mouth-ganglia, the mesosomatic and metasomatic remaining separate and distinct. The number of ganglia which have fused may be observed by examination of the embryo, in which it is easy to see indications of the individual ganglia or _neuromeres_, although all such indication has disappeared in the adult; thus the {53}infra-oesophageal ganglia of the cray-fish have been shown to be constituted of six prosomatic ganglia. In Fig. 25 I give figures of the central nervous system (with the exception of the abdominal or metasomatic ganglia) of Branchipus, Astacus, Limulus, Scorpio, Androctonus, Thelyphonus, and Ammocoetes. In all the figures the supra-oesophageal ganglia are lined horizontally, and their nerves shown, viz. optic (lateral eyes (II) and median eyes (II[prime])), olfactory (I) (first antennæ, camerostome, nose); then come the prosomatic ganglia (dotted), with their nerves (A) supplying the mouth parts, and the second antennæ or cheliceræ; then the mesosomatic (lined horizontally), with their nerves (B) supplying respiratory appendages. These figures show that the concentrated brain mass around the oesophagus of an arthropod which has arrived at the stage of Astacus, is represented by the supra-oesophageal ganglia and the fused prosomatic ganglia.