A small, aberrant subclass of Crustacea within the class Maxillopoda. These animals are cryptic meiobenthos (small benthic animals ranging in size between macrofauna and microfauna), and were first found living in the interstices of the intertidal sands at Nobska Beach, Woods Hole, Massachusetts, in 1939. In subsequent years, related species were uncovered along the Atlantic coast of Europe and in the Mediterranean, as well as along the northwest coast of Africa. Later still, collections were made in southern Africa. In all, some eight species were assigned to the single genus Derocheilocaris. At first, the evolution of mystacocarids was thought to be relatively recent and linked with the post-Cretaceous opening of the Atlantic. However, a second genus, Ctenocheilocaris, was discovered along the coast of Chile, and then several species of that genus came to light in Brazil. The definitive end of the circum-Atlantic scenario for mystacocarid evolution ended, though, when a fifth species of Ctenocheilocaris was described from the coast of Western Australia, near Perth. Mystacocarids are now perceived as a relict group of ancient origins.