polychaetes
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Hesionidae, Hesionides and Microphthalmus

Description

Hesionidae, unidentified

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.

Identification tips

Recognising the family
Hesionids range from large to very small interstitial worms. Carefully preserved specimens have numerous long anterior and dorsal cirri, but these are very fragile and are typically missing on preserved specimens. [NB that the enlarge anterior cirri are termed tentacular cirri in much of the literature, and in the family key in this CD publication.] The cephalisation of segment 1-3 up to 1-5, with differentiated dorsal and ventral cirri and reduced parapodia and chaetae, is characteristic, as is the presence of a pair of biarticulated, anteroventral palps. Nereididae have 3 or 4 pairs of tentacular cirri on the first two visible segments, but unlike hesionids have compound notochaetae. Phyllodocidae have 2 to 4 pairs of tentacular cirri but unlike hesionids have flattened leaf-like dorsal cirri and a single pair of eyes. Syllids have a characteristic proventricle and simple palps.

Distinguishing species
Colour may be useful in distinguishing species especially among live or freshly preserved specimens. Other useful characters are the papillae on the pharynx, presence or absence of chaetae in the notopodia, and the number, arrangement and length of anterior and regular dorsal cirri. However, identification often relies on specimens in good condition - many specimens from benthic surveys are unidentifiable unless the local fauna is very well known (in Australia it is not).

 

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