1 In Chelonia, the lower part of the shell, which is two-layered, consisting of bony plates corresponding to part of the shoulder girdle and abdominal ribs of primitive reptiles, covered by horny epidermal plates. It is joined to the carapace by a bridge between the legs. In some families it is hinged.
2 In some insects, a physical gill made up of hydrofuge hairs. Insects which use a plastron when diving under water are covered with a thick felt of hairs which are bent over at the tips. The hairs trap a layer of air which is continuous with the tracheal system, and surrounds the animal.
3 In Echinodermata, a flattened area between the mouth and anus, formed by the modification of the posterior interambulacrum. It is densely covered with spines.
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