Planarians are considered to be among the most primitive animals which developed the central nervous system (CNS). To understand the origin and evolution of the CNS, we have isolated a neural marker gene from a planarian, Dugesia japonica, and analyzed the structure of the planarian CNS by in situ hybridization. The planarian CNS is located on the ventral side of the body, and composed of a mass of cephalic ganglions in the head region and a pair of ventral nerve cords (VNC). Cephalic ganglions cluster independently from VNC, are more dorsal than VNC, and form an inverted U-shaped brain-like structure with nine branches on each outer side. Two eyes are located on the dorsal side of the 3rd branch and visual axons form optic chiasma on the dorsal-inside region of the inverted U-shaped brain. The 6th–9th branches cluster more closely and form auricles on the surface which may function as the sensory organ of taste. We found that the gross structure of the planarian CNS along the anterior-posterior (A–P) axis is strikingly similar to the distribution pattern of the “primary” neurons of vertebrate embryos which differentiate at the neural plate stage to provide a fundamental nervous system, although the vertebrate CNS is located on the dorsal side. These data suggest that the basic plan for the CNS development along the A–P axis might have been acquired at an early stage of evolution before conversion of the location of the CNS from the ventral to the dorsal side.
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