In the copepod genus Pontellina the development mainly of the fifth legs is pursued from their first appearance in the stage III copepodid to the adult. Specimens were employed with appendages in which the subsequent ontogenetic stage was sufficiently developed to trace the transformation
of the legs from one molt stage to the next. In the adult female not all elements on the two sides of the symmetrical fifth pair of legs are derived from corresponding ancestors. The appreciable ontogenetic changes in the male fifth pair of legs are clarified. Because the fifth legs are diagnostic
and serve in the identification of the species and because their modifications are sexually controlled, details of morphological changes during their maturation may be of considerable interest to the phylogeny of the genus.
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