The carapace of a typical cumacean is composed of several fused dorsal head parts and the first three somites of the thorax. This carapace encloses the appendages that serve for respiration and feeding. In most species, there are two eyes at the front side of the head shield, often merged into a single eye lobe. The five posterior somites of the thorax form the pereon. The pleon (abdomen) consists of six cylindrical somites.
The first antenna (antennula) has two flagella, the outer flagellum usually being longer than the inner one. The second antenna is strongly reduced in females, and consists of numerous segments in males.
Cumaceans are mainly marine crustaceans. However, some species can survive in water with a lower salinity rate, like in brackish water (e.g. estuaries). In the Caspian Sea they even reach some rivers that flow into it. Few species live in the intertidal zone.
Most species live only one year or less, and reproduce twice in their lifetime. Deepsea species have a slower metabolism and presumably live much longer.
Cumaceans feed mainly on microorganisms and organic material from the sediment. Species that live in the mud filter their food, while species that live in sand browse individual grains of sand. In the genus Campylaspis and a few related genera, the mandibles are transformed into piercing organs, which can be used for predation on forams and small crustaceans.
Many shallow water species show a diurnal cycle, with males emerging from the sediment at night and swarming to the surface.
Cumaceans are a clear example of sexual dimorphism: males and females differ significantly in their appearance. Both sexes have different ornaments (setation, knobs, and ridges) on their carapace. Other differences are the length of the second antenna, the existence of pleopods in males, and the development of a marsupium in females. There are generally more females than males, and females are also larger than their male counterparts.
Cumaceans are epimorph, which means that the number of body segments doesn't change during the different developmental stages. This is a form of incomplete metamorphosis. Females carry the embryos in their marsupium for some time. The larvae leave the marsupium during the so-called manca-stadium, in which they are almost fully grown and only miss their last pair of pereopods.
Cumaceans belong to the superorder of Peracarida, within the class of Malacostraca. The order of Cumacea is subdivided into 8 to 11 families, and about 120 genera. The families most marine zoologists recognize are: