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The current knowledge of brachyuran crab morphology is reviewed and its bearing on the classification of the group is examined. Brachyuran classification in the past has been based entirely on adult characters, but since adult crabs exhibit adaptations for a wide range of life styles some of the proposed groupings have resulted from a failure to recognize convergence. Since the zoeal stages of all crabs are adapted to the same mid-water habitat, a classification based on larval characters should be largely free from such problems.

Attempts to categorize crab zoeae in the past have generally failed to produce groupings which correspond closely to those based on the adults. In this paper, by using more characters than has usually been the case, and particularly the detailed setation of the mouthparts, a close correspondence with the adult groupings is demonstrated, at least at the family and sub-family level. At a higher taxonomic level the zoeal groups are often at variance with the widely accepted adult ones and strongly suggest the need for a fairly radical rearrangement. The major groups indicated by the zoeal features correspond most closely with those suggested by Guinot (1978) in her proposed extensive revision of brachyuran systematics. However, in both cases the major groups are not natural ones since they tend to identify grades of evolution rather than phylogenetic lines.

A key for the identification of zoeae to family or sub-family is provided, together with an extensive bibliography.