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Sigalionidae
Description
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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.
Description | Identification
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