Worms, Annelida 411 p lant mant pe 1st lant p p 1st mant tc 1st gi dc (b) (c) (a) pr p no bo pr 1st sap pe pe 1st gi 1st (e) (f) (d) Figure Polychaete anterior ends (a) Eunicid, dorsal view (b) Flabelligerid, dorsal view (c) Hesionid, dorsal view (d) Opheliid, lateral view (e) Orbiniid, dorsal view (f) Oweniid, lateral view Modified from Rouse GW and Fauchald K (1997) Cladistics and polychaetes Zoologica Scripta 26(2): 139–204 The Nervous System The nervous system consists of a dorsal brain connected by a double nerve ring running around the esophagus to double ventral nerve chords The anterior part of the brain is located in the prostomium, but penetrates well into the peristomium or even into the next several segments in many species In most groups the two parts of the ventral nerve chord are wholly or partially fused into a single entity; each segment always contains a pair of ganglia from which the segmental nervous system runs to the parapodial muscles, nephridia, gonads, and circulatory system The gut is invested from a separate set of nerves from the lower part of the brain (stomatogastric nerves) Antennal nerves run from the anterior part of the brain, and a pair of nerves associated with the posterior part of the brain runs to the palps Indeed, Orrhage has found a pattern in the emergence of nerves and internal connections in polychaete brains, showing that the limited number of nerves can be identified from one group to the next based on position and function The ventral nerve chord may contain two or more giant fibers associated with rapid retraction into burrows or tubes consisting of very thick nerves running from the anterior end innervating the longitudinal muscles directly with only a minimum of synapses Other motor fibers are segmentto-segment loops passing along undulatory locomotory waves The clitellates have a nervous system resembling the one present in the polychaetes, In other annelids the main nervous system resembles the one present in the polychaetes, but segmented nature of other nerves is absent