Giesbrecht, 1892
Females
Pd1 separated from Ce by faint line representing partially fused joint, Pd4 and 5 completely fused. R single, spiniform. Supralabrum highly prominent with rows of long stiff hairs, lies anterior to labrum. Ur of 4 somites. Gns symmetrical, or asymmetrical. Appendicular caudal setae highly developed, straight, smoothly curved, or geniculated, and usually longer than marginal caudal setae. A1 with extremely long seta on each of segments 3, 7, 9, 14, 18, 21 and 24; segments 8 and 9 fused. A2 Coxa with 1 seta, Basis with 2 setae, Enp1 with 2 setae and Enp2 with 8 setae on inner and 6 setae on outer lobe; Exp 7-segmented, Exp1 lacking setae, Exp2 with 1 (or lacking seta) marginal seta. Md Enp1 with 1 seta, Enp2 with 9 terminal and 1 appendicular seta (sometimes this seta absent). Mx1 Li1 with 9 terminal setae, 1 anterior seta and from 1 to 3 posterior setae, Li2 has 2 setae (sometimes 1 seta, or both setae absent), Li3 with 1 seta, Enp1 (consisting of 2 fused segments) has 1-7 setae, Enp2 with 3 setae, Exp with 11 setae and Le1 with 5-9 setae. Mx2 very strongly built; Li1-Li5 with 1 short and 2 long setae each; terminal setae of Li1-3 or Li1-4 armed with long spinules in addition to dense rows of short spinules; terminal part of Mx2 with 6 long setae. Mxp very strongly built; Coxa with 1 seta (this seta may be absent), then with groups of 2, 3 and 3 setae from proximal to distal; Enp1-5 with 4, 3, 2, 3 and 4 setae; medial edge of Basis armed either with short spinules only, or with both short and long spinules. P1 End with usually well-developed outer lobe; two first P1 Exp segments usually unseparated, Exp1 with outer spine that is usually very small or missing, Exp2 with outer spine and Exp3 with outer spine.
Spine and seta formula: Spine and seta Euchaetidae.
P5 absent.
Males
Similar to female, but appendicular caudal setae much less developed than in female. A1 with 8-10 segments completely fused; segments 12 and 13 partially fused; long seta found on each of segments 3, 9, 14, 18, 21, and 24. Setation on A2, Md, Mx1, and Mxp and sometimes even limb itself (Mx2) reduced compared with female. P5 large and complicate. Male left P5 Exp2 extends distally into serrated lamella (sl) and with digitiform process (dp); Exp3 with hairy tubercle (ht) proximally and distal groups of hairs; segment extended beyond these spines into spine-like process of various lengths or vestigial.
The family includes genus Euchaeta Philippi, 1843, and genus Paraeuchaeta Scott, 1909. Recently a revision was made of this family (Park, 1995). The genus Valdiviella Steuer, 1904 is usually also included in this family, but is distinct from Euchaetidae: two-pointed R; the length of setae on A1 (setae on corresponding segments shorter), A2 (Coxa without seta), Mx1 Li2-Li3 with 2-3 setae; Mx2 Li5 with 1 of setae stronger than others, spine-like; also subdivision of Exp and Enp segments of P2-P4 may be incomplete. P5 of males similar to these of Undeuchaeta in the Aetideidae; serrated lamella and digitiform process absent in left P5 Exp2. Until a complete revision of Valdiviella is made the genus is placed temporarily in the Euchaetidae.