(Figs 28–31)
Type species. Halacarus drygalskii Lohmann, 1907.
Adults. In female and male GP and AP fused. Female GA with 5–15 pairs of slender pgs, zero to one pair of sgs and three pairs of large, tube-like gac (Fig. 28; Newell 1984: fig. 201). Ovipositor at rest extending beyond GO. Genital spines slender, number not known. Male GA with more than 100 pgs densely arranged on slightly raised area around GO. Male GO somewhat smaller than that of female; GO in both male and female in ventral position. Each genital sclerite with five short sgs, often two anterior pairs slightly separated from three posterior ones, fourth pair (sgs- 4) stump-like (Newell 1984: fig. 184). Male with three pairs of large internal gac (Fig. 29). AE of adults without epimeral pores.
Juveniles. One larval and two nymphal stages known. In nymphs GP separated from AP. Deutonymphal GP with two pairs of large internal gac, one pair of pgs and two pairs minute sgs (Fig. 30; Bartsch 1992 d: fig. 9). Protonymphal GP very small; with one pair of large gac (Fig. 31); pgs and sgs not developed. AE of nymphs without, of larvae with pair of epimeral pores (Bartsch 2004 a: fig. 14).
Remarks. Almost 20 species are known, often they are found associated with hydrozoans in polar waters but are present also in warm-temperate areas. Records are from shallow water to the deep sea (Bartsch 2009 a).