Willemoes-Suhm, 1875
Large animals (to 140 mm) (Cystisoma fabricii 1, Cystisoma longipes 1). Cuticle thin, parchment-like, completely transparent; musculature very weak. Head very large, rounded dorsally, ventral surface flat with dentate marginal carinas (Cystisoma pellucida 1, Cystisoma fabricii 2). Two other rows of "ventral" denticles forward of mouth parts. "Top" (anterior) denticle in these rows largest, present in all species; the second ("glandular") denticle is also present in all species. This denticle is a rudimentary ANT II, as indicated by the presence of a gland opening at its base (Stebbing, 1888). Other ventral denticles may number 0-6, sometimes different in right and left rows. Eyes very large, oval, practically without pigment, situated on dorsal surface of head. ANT I with reduced peduncle ART, 2-articulate flagellum, with first ART long and slender, second strongly reduced. ANT II attached to ventral surface of head, strongly reduced, spine-like (see above). MD without palp, with one (central, Cystisoma magna 2) or few (central and lateral, Cystisoma longipes 2) denticles on its body. Pereonites and pleonites with short mid-dorsal spines/carinas. First two to four pereonites may be fused. Coxae fused with pereonites. P with serrate margins, P I and II chelate (Cystisoma pellucida 2). In adult females distal part of P VII developed into "eggs pincers": distal end of elongate ART 6 expanded with rounded excavation (egg size), dactyl strongly curved, able to fix egg in this excavation. UR II absent, other UR with fused basipodites and endopodites. Deep-water animals.
Only one genus in family:
Genus Cystisoma Guérin-Méneville, 1842
Cystisomatidae show specific development of physosoma stage in young (with long, but weak, parchment-like spines on body, see Physosoma); length of physosoma: to 15 mm.