(Alder & Hancock, 1846)
This uncommon species may reach 65 mm in extended length and bears a superficial resemblance to the common 'sea lemon' Archidoris pseudoargus. The latter species may reach 120 mm, however, and there are differences in the mantle coloration, which is less vivid in G. planata and usually has a brown or purplish brown dominance. Furthermore, there are numerous (up to twelve) stellate pale areas forming a loosely paired series on the dorsal mantle, and these represent aggregations of acid gland markings. Their secretions are strongly acidic, contrary to the neutral mantle of Archidoris. Finally, the head in G. planata bears conspicuous finger-like oral tentacles, lacking in Archidoris. The propodium is bilaminate, the anterior lamina notched in the midline. Juveniles are usually paler in colour and more flattened.
This shallow water species feeds upon sponges.
It has been recorded from Norway to the French Atlantic coast (Distr. G. planata), and in the Mediterranean Sea (Naples), to 45 m.