The posterior patagium, the fold between trunk and inner surface of the upper arm, is stretched by the metapatagialis muscle, which is composed of slips from the serratus, superficialis, latissimus dorsi and the expansor secundariorum muscles.
The larger shrimp-like crustacea are generally known as "prawns," the name being especially applied in Britain to the species Leander serratus, formerly called Palaemon serratus, which is highly esteemed for the table.
The dark areas are dominated by serrated wrack Fucus serratus.