The nervous system is of primitive type resembling that of coelenterates and echinoderms.Nervous layer is thickened along definite strands to form synapses with the processes of nerve cells.Nervous layer is thickened along definite strands to form two main nerve cords, one mid-dorsal and other mid-ventral,which run along the entire length of the trunk. Ventral cord extends upto collar-trunk septum where it is connected with the dorsal cord by a circular strands, called circumenteric nerve ring. Dorsal cord extends anteriorly upto the base of proboscis whereit is connected with another circular strands called anterior nerve ring.Dorsal cord leaves the epidermis and traverses the collar coelom as collar cord.The collar cord contains a cavity called neurucoel.
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