The family was long limited to the single genus Nebalia (Leach), and the single species N.
There are also now two more genera, Paranebalia (Claus, 1880), in which the branchial feet are much longer than in Nebalia, and Nebaliopsis (Sars, 1887), in which they are much shorter.
All known species are parasitic on the Crustacean Nebalia; Seison Claus; Paraseison Plate.
Drilophagus and Albertia are parasitic on the surface or within the gut of Naid Oligochaete worms: Seisonaceae are ectoparasitic on the Crustacean Nebalia, Proales werneckii forms galls within the Conferva Vaucheria, and P. parasita infests the central jelly of the Phytoflagellate Volvox; P. petromyzon is a frequent commensal in the gill cavity of some Cladoceran Crustacean Eurycereus lamellatus.
2, Nebalia bipes (one side of 5, Branchipus stagnalis: a, adult carapace removed).
5, 3) and in Nebalia (fig.
In the primitive Malacostraca the gills were probably, as in the Phyllopoda and in Nebalia, the modified epipodites of the thoracic limbs, and this is the condition found in some Schizopoda.
There is considerable reason for believing that the Ceratiocaridae, which are found from the Cambrian onwards, were allied to the existing Nebalia, and may possibly include the forerunners of the true Malacostraca, but nothing is definitely known of their appendages.
For the Malacostraca, it is generally admitted that the Leptostraca (Nebalia, &c.) provide a connecting-link with the base of the Phyllopod stem.