. A manual of zoology. Zoology. of nervous elements. In the medusce such a place is the rim of the bell; consequently a stronger nerve-cord much richer in ganglion cells is found here. This, as well as the nerve-ring and the five radial nerves of echinoderms, may be called a central system, thereby distinguishing the rest of the nervous network as the peripheral nervous system. Ganglionic Central Nervous System.—Numerous transitional forms lead to the ganglionic central nervous system of the worms, molluscs, and arthropods (fig. 77). The central nervous system here consists of two or more gang
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. A manual of zoology. Zoology. of nervous elements. In the medusce such a place is the rim of the bell; consequently a stronger nerve-cord much richer in ganglion cells is found here. This, as well as the nerve-ring and the five radial nerves of echinoderms, may be called a central system, thereby distinguishing the rest of the nervous network as the peripheral nervous system. Ganglionic Central Nervous System.—Numerous transitional forms lead to the ganglionic central nervous system of the worms, molluscs, and arthropods (fig. 77). The central nervous system here consists of two or more ganglia; each ganglion being a bunch of regularly arranged nerve-fibres and ganglion-cells. The former con- stitute the centre of the mass, and, since they cross in all directions giving off branching den- drites, they appear like fine granulations. The ganglion-cells, on the other hand, collect in a thick layer around the granules. The peripheral nerves, and also the commissures, the cords con- necting similar ganglionic masses, extend out- wards from the ganglia. Supraoesophageal (or Cerebral) Ganglia.— Since most animals are symmetrical, the ganglia occur in pairs; left and right ganglia correspond to one another and are connected simply by a cord of nerve-fibres, the transverse commissure. Of most constant occurrence are two ganglia, which lie dorsally above the pharmx, and hence are called the supra-cesophageal or cerebral ganglia. If other ganglia occur, they lie ventrally and below the digestive tract {ventral nerve-cord). In the Ladder Nervous System of annelids and arthropods (fig. 78), numerous pairs of ganglia (in the example before us, nine) lie in serial order on the ventral side of the animal, and are connected by longitudinal commissures {con- nectives), and also by transverse commissures connecting the left and right ganglia. The first pair of the series is the infra-cesophageal ganglion, which sends out connectives right and left, surrounding the pharynx, to t