Abstract
The structure of the coxal gills and coxal plates of the semi-terrestrial beachflea Orchestia gammarellus (Pallas) (Crustacea: Amphipoda: Talitridae) is described in relation to their possible use for aerial gas exchange and ion exchange. Anatomical evidence is presented to support the hypothesis that the medial surface of the coxal plates functions as an extrabranchial aerial gas-exchange site in O. gammarellus. Thus, the effective diffusion distance across the medial (or inside-facing) surface of O. gammarellus coxal plates (mean±SD=5.4±0.3 μm; n=9, cuticle thickness 4.4±0.5 μm, n=21) is only a third of the equivalent distance across both the coxal gills (18.4±6.0 μm, n=10; cuticle thickness 1.7±0.6 μm, n=7) and the lateral (or external) surface of the coxal plates (19.4±0.7 μm, n=5; cuticle thickness 8.7±0.8 μm, n=7). Chloride-ion-permeable areas were located using a silver-staining technique. All ten coxal gills appeared to be equally permeable to chloride ions after examination with a light microscope. However, the coxal plates and the rest of the integument do not appear to be chloride-permeable.
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