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ABSTRACT
We discuss some of the developmental differences between lancelets and craniates that caused classical workers problems in phylogenetic arguments concerning the origin of chordates. Modem insights into developmental biology argue against overemphasis on topographic similarity during development in the deduction of phylogenetic distance. We identify processes underlying lancelet development which provide a reasonable basis for homology with the vertebrate Bauplan and argue that lancelet larval asymmetry is a unique adaptation associated with the feeding mechanism. Hence, the unusual feature of lancelet asymmetry is an unreliable guide to the ancestral developmental pattern of chordates as a whole. The overall pattern of metameric development in lancelets is seen as close to that of craniates and as distant from the Bauplan of any other group of deuterostomes.