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A description is given of a new species of pontoniinid prawn commensal with bivalves of the genus Placuna. The species is close to members of the genus Comhodytes but differs in extreme flattening and consequent modifications which can be taken as adaptations to its peculiar environment. In behaviour and morphology it differs strikingly from the well known bivalve commensal Anchistus custos Forskal but is equally well adapted for its' commensal life. The presence of filtratory mouthparts represents an unusual adaptation. Redescriptions and morphological comments are also given for two little known commensal porcelain crabs. In these also it is possible to explain many of the morphological peculiarities in functional terms.

Summary

Accounts are given of the structure and some aspects of the biology of three commensal crustaceans from Singapore. One of these represents a new genus of prawns of the subfamily Pontoniinae; the others are little known members of the family Porcellanidae.

It is suggested that the apparent rarity of these and other tropical commensal decapods is probably a spurious phenomenon resulting from inadequate collecting methods.

The functional taxonomy of these commensals is discussed and it is suggested that many of their structural peculiarities can be interpreted as adaptations to their mode of life.