The absence of a head shield is the basis for the name of this order. The mantle cavity is greatly reduced and enclosed by the parapodia, or flap-like lateral outgrowths of the foot-margins. The shell is internal and all species have a radula and gizzard plates. This group of opisthobranchs have been observed breeding in large groups, more specifically engaging in chain copulation. The group is primarily herbivorous, feeding principally upon red and green algae.