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How do leeches swim?

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annelid *The structure of polychaetes.* (Left) Free-moving polychaetes. (A) *Neanthes,* (B) *Nereis.* (Right) Tube-dwelling (sedentary) polychaetes. (C) *Amphitrite,* (D) *Sabella.*

When swimming, leeches maintain their dorsoventral muscles in a contracted state. This posture allows them to produce undulations of the body through waves of contraction generated by their longitudinal muscles.1

While leeches are well-known for their "contract-anchor-extend" method of crawling along surfaces using their anterior and posterior suckers, their swimming mechanism relies on these rhythmic muscular undulations to propel them through the water.12