The leaf-shaped body may reach 45 mm in length and the lateral parapodial 'wings' may be outstretched or held over the so that they nearly meet in the midline. The body colour varies with the diet, from green to red. Relatively constant features are, however, tiny glistening red, blue, and green specks. White patches may also be found, especially at the edges of the , and black markings are sometimes seen on the head and elsewhere. Ramifying diverticula of the and of the are often visible through the skin of the and even of the and other parts of the head. The is -lateral. The are prominent and inrolied. The characteristic make their appearance at a body-length of 3 mm; younger individuals may lack them ( ).
This common species occurs on a variety of shallow-water algae, especially Codium and Cladophora . Despite the graceful form of the body and the wide and mobile , swimming has never been observed."
Marine Species of the North Sea: http://www.species-identification.org/species.php?species_group=mollusca&selected=beschrijving&menuentry=soorten&id=905