Arrow worms or bristlemouth worms
Slender, tiny and transparent, the planktonic arrow worms present viewing problems even though they may be extremely abundant when in “bloom.” Each species possesses a pair of lateral fins and a tail fin, but the worms swim by repetitive flexing of the central body area. This distinctive motion is an eye-catching hint of its presence. The most obvious anatomical feature of an arrow worm is a formidable pair of bristle-bearing “jaws” adjacent to its mouth.
Eighteen species of arrow worms are found in the marine waters of the Pacific Northwest.

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Arrow Worms