Mystacocarids have shortened, vermiform bodies, with about one-third of the total body length being taken up by the head. The trunk consists of a very short maxillipedal somite, followed by nine more-or-less similar somites, the first four of which bear reduced appendages. The last five trunk somites (abdomen) do not have appendages. The trunk ends in a large telson (=anal) somite, which has a pair of simple, terminal, furca-like appendages. All trunk somites and the posterior section of the head have dorsolateral toothed furrows of unknown function. The head appendages are the most conspicuous aspect of mystacocarids. The antennules are very long, about half the length of the body. The antennae, mandibles, and both pairs of maxillae are composed of large circular segments whose shape is maintained by fluid pressure. The antennae and mandibles are biramous, whereas both pairs of maxillae are uniramous. The maxillae are armed with long setae on their inner margin.
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