Figure 1

Overview of the central nervous system of Lineus viridis. a: Head of a living specimen of Lineus viridis (Light microscopy, dorsal view) slightly squeezed, showing the location of the reddish brain (br) and the eyes (e). b: Schematic drawing (dorsal view) of the anterior nervous system of L. viridis (the neuronal somata and the cephalic nerves of the ventral part of the brain were omitted). The ventral commissural tract (vct) connects the two ventral lobes (vl) of the brain, while the dorsal commissural tract (dct) connects the two dorsal lobes (dl). Posteriorly, the ventral lobes of the brain merge with the medullary cords (mc). Medially, the esophageal nerves (en) arise from the ventral lobes. A dorsal nerve cord (dnc) arises from the dorsal commissural tract. Cephalic nerves (cn) extend from the dorsal lobes towards the anterior tip of the animal. The paired proboscidial nerves (pn) originate from the ventral commissural tract. c-f: Cross sections (Azan staining) of the head of L. viridis showing the structures of the central nervous system as shown in b. c: The brain (br) is composed of neuropil (n) which is surrounded by neuronal somata (s). The brain encloses the rhynchocoel (ry). d: Paired proboscidal nerves (pn) run the full length of the proboscis inside the proboscis musculature. c-f: The cephalic slits (cs) narrow each into a canal (ca) which leads to the cerebral organ (co). The cerebral organ (co) is attached posteriorly to the inferior branch of the dorsal lobes of the brain (idl). a anterior; bv blood vessel; d dorsal; ec esophageal commissure; in inner neurilemma; mo mouth opening; on outer neurilemma; p posterior; sdl superior branch of the dorsal lobe; v ventral. Scale bars: a = 500 μm; c-f = 200 μm.