Fig. 5

3D- model of the anterior cns (A), histological cross sections (5 μm), Azan staining, frontal (B) to caudal (D) of Owenia fusiformis. a: the central nervous system (cns, gray: neuropil, blue: somata, only drawn on the left) is composed of a ring shaped brain (br). Eyes (ey) are located ventro- laterally on the brain ring. The brain ventro- caudally gives rise to paired medullary cords (mc) which fuse in their further course to a single cord. The two medullary cords are occasionally connected by ventral commissures (co). The stomatogastric system is surrounded by a nerve plexus which arises from two nerves connected to the ventral part of the brain ring. ca: caudal; fo: frontal. b: the brain (br) is located within the epidermis (ep). bl: basal lamina; ca: caudal; eso: esophagus; ey: eye; fo: frontal. c: A stomatogastric plexus (sgp) innervates the stomatogastric system. An epidermal plexus (epp) is present posterior to the brain. The medullary cords (mc) are ventrally connected by commissures (co). The medullary cords are composed of a neuropil (np) and somata (so) which are located ventrally to the neuropil. d: Posteriorly the two cords fuse in chaetiger one to a single medio- ventrally located medullary cord (mc). ch: chaetae