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Sigalionidae: Sthenelanella sp.

The prostomium is anteriorly blunt or rounded, and the peristomium reduced to lips around the mouth. Paired lateral and a median antennae are present in most taxa; the latter may be absent. Although the lateral antenna originates on the prostomium it may be fused to the parapodia of the first segment. The palps are ventral and tapering, and are without articulations; they are fused basally to the parapodia of the first segment. Nuchal organs are present. Longitudinal muscles are grouped in bundles, and segmentation is distinct. The first segment curves around the head, and is fused basally to the prostomium. Parapodia and two pairs of tentacular cirri are present on the first segment. Parapodia are biramous and the neuropodia are longer than the notopodia. Dorsal and ventral cirri are present. At the anterior end, dorsal cirri alternate with elytra. Some taxa have tapering dorsal cirri on segment 3, whereas others have neither elytra or dorsal cirri on segment 3. Elytra are present on every segment of the posterior end. True branchiae are absent. The ventral surface may be smooth or papillated. One pair of pygidial cirri is present. Lateral organs and dorsal cirrus organs have not been observed. The buccal organ is an axial muscular eversible pharynx; terminal papillae and two pairs of dorso-ventrally oriented jaws are present. The gular membrane is absent, and the gut has segmentally arranged side branches (caeca). Mixonephridia are present in most segments. The circulatory system is closed and lacks a heart body. Aciculae are present. The parapodia bear compound chaetae which taper to fine tips or are distally dentate or falcate. The chaetal shafts and appendages are joined by a single ligament. Other chaetae are variously ornamented capillaries. In many taxa, slender, digitiform extensions from the body wall project under the elytra; these presumably have a respiratory function.

The above description is taken from Hutchiings (2000), which in turn is based on that of Fauchald & Rouse (1997).

Identification tips

Recognising the family
Sigalionids are long bodied scale worms, with well developed head with many appendages and with dorsal elytra covering the dorsum to varying degrees. In one genus these elytra are encrusted with sand grains, and some species are tubiculous. They differ from all other scale worms except Pholoidae in having compound chaetae.

Sigalionids are most likely to be confused with one of the other families that also have dorsal rows of scales. The quick guide to scale worms should help separate these 6 families.

Distinguishing species
Species of Sigalionidae are distinguished by the structure of the head and the relative development of the antennae and the development of eyes. The elytra are variously ornamented and their margins may be smooth or have papillated lateral margins. The distribution and type of chaetae present is also important as are the distribution and arrangement of papillae and basal bracts and their ornamentation on both notopodia and neuropodia.


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