Radial nerve cord ( Galen, c173 ) : A topographic division that is a differentiated linear zone of the nerve net in Echinoderms with periodic small ganglia (Galen, c173) along its length, and extending into each of the (most commonly five) arms from the circumoral nerve ring. There is no convincing evidence that the Echinoderm nervous system (Monro, 1783) is divided into central nervous system (Meckel, 1817) and peripheral nervous system (Meckel, 1817); see Cobb (1995, p. 411). A cord segment between two adjacent ganglia is called a longitudinal communicating branch.