SIMILARITIES AND DIFFERENCES IN LANCELET AND CRANIATE NERVOUS SYSTEMS

The central nervous system and some sense organs of lancelets show sim ilarities to and differences from those of craniates. Lancelets and ma ny craniates share such cellular elements in the spinal cord as the Ro hde interneurons. Recent tracing experiments indicate that this simila rity may extend to large paired cells in the hindbrain region, includi ng the Muller cells and perhaps the Mauthner cells. Similarity between lancelets and craniates may end at the hindbrain/midbrain boundary, w hich is possibly demarcated by the expression of the Engrailed gene. W hile most craniates develop some ocular motoneurons adjacent to this a rea of gene expression, lancelets appear to lack such motoneurons. Top ological similarity in this region of the neuraxis apparently does not extend to exact replication of cellular fate in the two groups. Simil arities in the central projections of primary sensory cells of lancele ts and of the placodal cranial ganglia in craniates are discussed in r elation to their possible homology and to the potential role of neurot rophins in their development. The recently discovered autofluorescent cells at the anterior end of the lancelet neuraxis lack homologues in craniates and may represent an autapomorphic character acquired by lan celets but not inherited by the craniates.