Hesionides & Microphthalmus are recognised as forming one (or two) groups separate from Hesionidae. Descriptions of both groups are provided separately:
Hesionidae sensu stricto
General features. Body shape vermiform; segments numerous (more than
about 15); regionation absent. Pygidium simple ring or
cone. Pygidial appendages present; one pair of cirri, or one pair of
cirri and single medial papilla.
Head & head structures. Head discrete and compact, dorsal to
mouth. Prostomium rounded to oval (anteriorly truncate). Facial tubercle absent,
or present. Eyes present (rarely absent); two pairs; situated on prostomium;
compound with lenses. Prostomial antennae present; paired arising
anterolaterally, or include paired anterolateral ones and single medial one;
smooth. Palps paired, or absent (rarely); unarticulated, or bi-articulated;
ventrolateral. Nuchal organs indistinct paired dorsolateral patches. Peristomial
ring absent.
Pharynx & pharyngeal apparatus. Foregut a muscular
axial pharynx; without jaws, or with one pair of lateral jaws; plate-like, or
fang-like; with a single tooth, or without teeth; surface smooth;
distal ring of papillae present (reduced in some
taxa).
Body segments & parapodia. Enlarged dorsal cirri and cirrophores on segment 1-3, 1-4 or 1-5; enlarged ventral cirri and distinct cirrophores on segment 1-3 or 1-4; internal aciculae present in at least some
enlarged anterior cirri. First chaetiger equals segment 4 or 5 (rarely 3), with neurochaetae only. Parapodia uniramous
throughout, or biramous with prominent neuropodial lobes; notopodial lobes
represented by at least one chaetal lobe, or absent; neuropodial lobes
represented by at least one chaetal lobe. Dorsal cirri more-or-less cirriform.
Ventral cirri present, distally or subdistally inserted on neuropodium. Branchiae absent.
Chaetae. Notochaetae absent, or present. Aciculae present; in both
dorsal and ventral positions (=noto- and neuroaciculae). Capillary chaetae
hair-like; hirsute-serrate. Forked notochaetae absent, or present; furcate
type; tines distinctly unequal in length. Spines absent. Hooks absent (present
in the notopodia of one genus). Compound chaetae present; appendage distally
curved (=falcate); appendage not canaliculated; appendage without hoods or
guards; joint distinctly asymmetrical (=heterogomph); joint effected by
ligament; shaft tapering slightly or evenly thick from emergence to joint;
shaft with camerated core. Compound falcigers present all chaetigers.
Tube & burrow. Tube absent or unconsolidated. Burrow traces
absent.
Hesionides & Microphthalmus
General features. Body shape vermiform; segments numerous (more than
about 15); regionation absent. Pygidium lamellate lobe(s)
(single or bilobed). Pygidial appendages present; one pair of cirri.
Head & head structures. Head discrete and compact, dorsal to
mouth. Prostomium rounded to oval (anteriorly truncate). Eyes absent, or
present; one pair; situated on prostomium; without lenses. Prostomial antennae
present; paired arising anterolaterally, or include paired anterolateral ones
and single medial one; smooth. Palps paired (resemble antennae); unarticulated;
ventrolateral. Nuchal organs indistinct paired dorsolateral patches. Peristomial
ring absent.
Pharynx & pharyngeal apparatus. Foregut a muscular
axial pharynx; distal ring of papillae present.
Body segments & parapodia. First segment with tentacular cirri.
Enlarged ventral cirri on segment 1-2 or 1-3. First chaetiger equals segment 3 or 4, with both notochaetae and neurochaetae. Parapodia uniramous
throughout; notopodial lobes absent; neuropodial lobes represented by at least
one chaetal lobe. Dorsal cirri more-or-less cirriform. Ventral cirri present,
cirriform or tapering. Branchiae absent.
Chaetae. Notochaetae present. Aciculae present; in both dorsal and
ventral positions (=noto- and neuroaciculae). Capillary chaetae hair-like (often
sickle-shaped); hirsute-serrate. Forked chaetae absent. Spines absent. Hooks
absent. Compound chaetae present; appendage distally curved (=falcate);
appendage not canaliculated; appendage without hoods or guards; joint distinctly
asymmetrical (=heterogomph); joint effected by ligament(s); shaft tapering
slightly or evenly thick from emergence to joint; shaft without distinct core.
Compound falcigers present in most or all chaetigers.
