Krill (euphausiids) are small shrimp-like crustaceans. They are partly transparent, except for their set of darkly pigmented eyes. Arrow worms are planktonic marine worms in the exclusive phylum Chaetognatha. They are transparent and arrow-shaped. Chaetognaths swim using their tail fin for propulsion and the body fins for stabilization and steering. Euphausiids are more pigmented and less streamlined than the chaetognaths. Also, krill have more noticeable eyes at their anterior end, which makes it easier to distinguish between the two organisms in this video.