EXCEPTIONS TO THE RULE OF DEVELOPMENT THAT ANTERIOR IS OLDER AMONG SERIALLY HOMOLOGOUS SEGMENTS OF POSTMAXILLIPEDAL LEGS IN COPEPODS

A survey of more than 250 species of copepods shows about 4% of the rami on postmaxillipedal legs have a segment that is formed later in development, and therefore is younger, than homologous segments on legs located on more posterior body somites. These younger segments are exceptions to the general rule of development that, among serially homologous segments, those found on anterior legs are older than their homologues on more posterior legs. Exceptions to this rule occur more frequently on leg 1 than expected, and appear to have originated more than once during copepod evolution. These younger segments more often are added during the terminal adult molt, which is an unusual developmental pattern for copepods.