Sars, 1902
Females
Total length 0.80 - 7.00 mm. Body relatively robust, Ce and Pd1 separated or weakly fused, Pd4 and 5 fused, sharply or weakly produced, with or without dorsolateral and/or ventrolateral processes. Ce rounded or pointed anteriorly, R produced ventrally with pair of filaments. Ur relatively short, of 4 somites; Gns with single paired gonopores and copulatory pores; gonopore(s) located ventrolaterally or ventrally, with or without opercular plate; copulatory pore sharing common opening with gonopore or separate with gonopore located ventromedially or posteriorly, rarely ventrally on right side; seminal receptacles usually paired, rarely left receptacle entirely lacking. Egg-sac present or absent. CR symmetrical or slightly asymmetrical, relatively short with up to 7 setae. A1 symmetrical or asymmetrical, longer on left than on right, 16-22-segmented, sometimes differing in fusion pattern and armature. A2 Coxa without seta; Basis with outer edge seta; Exp and Enp separated from Basis; Enp 2-segmented, segment 1 with 0-1 seta at midlength, segment 2 with 1-3 inner setae medially and 5 or 6 setae terminally; Exp indistinctly 6-10-segmented. Md gnathobase well chitinized, with 3 or 4 sharp teeth; Enp rudimentary, 1-segmented with 1 or 2 setae terminally or completely absent; Exp 5-segmented, segment 1 with normal or reduced setae, segments 2-4 with 1 seta each, segment 5 carrying 2 setae, one of which is sometimes vestigial. Mx1 Li1 0-6 spines and setae; Li2 with 1 seta or unarmed; Li3 represented by 1 seta which may be vestigial or absent; Enp bulbous, 1-segmented, with 0-3 setae or completely incorporated into Basis; Exp lobate, with 3 setae; Le1 with 5-9 setae. Mx2 well developed; Li1 with 1-3 setae (1 vestigial), Li2 with 1-2 setae; Li3 and 4 with 2 setae each; Li5 with stout spine, spinulose or bare; Enp 4-segmented with 1, 3, 2, 2 setae respectively. Mxp elongate, Coxa with 1 medial and 2 terminal setae; Basis with patches of setules or spinules and 2 setae medially; Enp 6-segmented, segment 1 almost fully incorporated into Basis with 1, 4, 4, 2-3, 2-3 , 4 - 2 setae, outermost terminal setae of segment 6 may be reduced. P1-4 distinctly 3-segmented, rarely with Enp segments of P1 incompletely fused.
Spine and seta formula: Spine and seta Arietellidae.
P5 variable, not natatory, almost symmetrical; Coxa and intercoxal sclerite separate or fused; Basis and Enp separate or fused; Enp with 0-4 setae; Exp 1-3-segmented or completely fused with Basis, carrying 0-5 spines and setae.
Males
Body similar to female, but Ur of 5 somites. Left A1 geniculate, 16-20-segmented, proximal segments often with row of setules along the posterior margins. Mouthparts and P1-4 similar to female or with slightly different armature of A2 Enp2 and Md Exp1. P5 variable, not natatory, almost symmetrical to strongly asymmetrical; Coxa and intercoxal sclerite fused to form common base or separate; right Basis sometimes fused with Coxa; right Enp 1-segmented, bulbous or absent; right Exp distinctly or indistinctly 3-segmented, segments 1 and 2 each with 1 seta on outer margin (rarely segment 1 unarmed), segment 2 with tuft of setules on inner distal angle of segment 2, segment 3 with 0-3 spines and setae terminally; left Enp 1-2-segmented, unarmed or completely absent; left Exp distinctly 3-segmented, segments 1 and 2 each with seta on outer margin, segment 3 with 1-3 spines and setae terminally.
There are now 11 genera in this family, of which genus Arietellus, genus Paramisophria, genus Paraugaptilus, and genus Pilarella occur in the South Atlantic:
Genus Arietellus Giesbrecht, 1892
Genus Campaneria Ohtsuka, Boxshall, and Roe, 1994
Genus Crassarietellus Ohtsuka, Boxshall, and Roe, 1994
Genus Metacalanus Cleve, 1901
Genus Paraugatiloides Ohtsuka, Boxshall, and Roe, 1994
Genus Paraugaptilus Wolfenden, 1904
Genus Pilarella Alvarez, 1985
Genus Paramisophria T. Scott, 1897
Genus Rhapidophorus Edwards, 1891
Genus Sarsarietellus Campaner, 1984
Genus Scutogerulus Bradford, 1969
Recently an excellent revision has been made of this family which describes new genera, some accommodating already known arietellids, which treats character transformations of the genital system and appendages in detail, and includes phylogenetic relationships based on a cladistic analysis (Ohtsuka et al., 1994). The above definition excludes Rhapidophorus Edwards, 1891 which is inadequately described. Arietellids are widely distributed from neritic (including cave-dwelling species) to oceanic waters and ranging vertically from the epipelagic to bathypelagic hyperbenthic layers (Ohtsuka et al., 1994).