Central nerve cord : For the invertebrate central nervous system (Meckel, 1817), a topographic division that is a longitudinal central nerve cord trunk with a series of more or less regularly spaced central ganglia along its course, and a central nerve cord trunk segment between two adjacent central ganglia called a central longitudinal communicating branch. For invertebrates, the central nervous system (Meckel, 1817) generally consists of one or more central nerve cords, with central transverse communicating branches-commonly referred to as commissures-typically interconnecting central ganglia between central nerve cords (e.g., a right and a left) and forming a ladder-like, or orthogon (see Reisinger, 1925, 1972), arrangement with rostral (Schulze, 1893) and caudal (Cleland, 1879) ends. The most rostral of the central ganglia is commonly called the invertebrate brain or cerebral ganglion. In annelids and arthropods the most rostral of the central ganglia generally lies dorsal (Barclay, 1803) to the digestive system and is called the supraesophageal ganglion, whereas the rest of the central nerve cord lies ventral (Schulze, 1893) to the digestive system. See Bullock & Horridge (1965).

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