Exopodites, epipodites and gills in crustaceans

Abstract

The structure of the outer parts of the maxillae and post-maxillary limbs is compared across the major crustacean groups. New anatomical observations are presented on the musculature of selected limbs of key taxa and general patterns in limb structure for the Crustacea are discussed. Exopodites vary in form but are typically provided with musculature, whereas epipodites and other exites lack musculature in all post-maxillary limbs. Within the Crustacea, only the Myodocopa possesses an epipodite on the maxilla. New evidence from developmental genetics, from embryology, and from new Palaeozoic fossils is integrated into a wider consideration of the homology of exites (outer lobes). This evidence supports the homology of the distal epipodite of anostracan branchiopods with the epipodite-podobranch complex of malacostracans. The evidence for the homology of pre-epipodites across the Crustacea is less robust, as is the evidence that the possession of a proximal pre-epipodite and a distal epipodite is the ancestral malacostracan condition. The widely assumed homology of the peracaridan oostegite with the pre-epipodite is questioned: little supporting evidence exists and possible differences in underlying control mechanisms need further exploration.