An oval sheet up to 7 cm (2.8 in) long, orange to warm brown, crossed by a scatter of fine broken white lines and edged all round by a narrow black margin. Color is the whole point: a flatworm has no shell, no spines and no speed, so it advertises instead, and the pattern says that swallowing this will be regretted.
The advertisement is largely honest. Polyclad flatworms carry toxins in the skin, some of them the same tetrodotoxin that makes a pufferfish dangerous, and fish that try one tend not to try a second. Several harmless nudibranchs copy the look, which only works because the original is genuinely unpleasant.
Underneath, the animal is a simple, efficient machine. A branching gut spreads through the body instead of a circulatory system, moving food to the tissue directly; the mouth is on the belly, halfway along. It hunts by smell, tracking sea squirts and bryozoans across the rubble at a pace that never looks like hunting at all.
- orange to warm brown body crossed by fine broken white lines
- narrow black margin all the way round
- up to 7 cm, oval and thin as a leaf
- glides over rubble rather than swimming
