Gáspár Jékely and colleagues' paper on page 395 literally takes the eye. It provides a neat dissection of how the two eyespots in the larva of a marine worm (Platynereis dumerilii) perceive light, and then how the larva responds with differential beating of cilia and a directed, helical swimming action (G. Jékely et al. Nature 456, 395–399; 2008).

Figure 1
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F. NEDELEC

Full size image

These stills come from a dynamic computer simulation, created from first principles, in the authors' Supplementary information. They show the movement of six virtual larvae that are attracted to a shifting light source; the simulation is initiated in frames 1 and 2, and shows the larval reaction when the light source moves to three different positions (frames 3, 4; then 5, 6; and finally 7, 8). The spiral trajectories traced by each of the organisms closely mirror the behaviour of real Platynereis larvae.

This apparently inefficient form of motion, conclude Jékely et al., is optimal for combining eyespot light detection with precise navigation. The authors also point out that at least some of the same principles probably apply to the many members of the zooplankton that swim in spiral fashion.

More about the paper can be found in 'Making the paper' (page xiii), and 'An eye for the eye' (page 304) provides a pictorial gallery of ocular evolution. The computer simulation can be viewed at http://tinyurl.com/59ah73.