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Trichobranchidae
Description
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The Trichobranchidae is characterised by the fusion of the
prostomium to the peristomium along the anterior margin of
the prostomium. The peristomium forms extended lips; the
upper lip may be expanded to form lateral lobes and the lower
lip may be greatly expanded to form a ridged, conical
eversible proboscis which may be completely retracted. The palps are represented by multiple
grooved buccal tentacles, which cannot be retracted into the
mouth. Nuchal organs are present in Trichobranchus species,
but not in Terebellides species; the condition is
unknown for other genera. Segmentation is
distinct. The first segment is fused to the head and lacks
parapodia and chaetae. The body is divided into two regions:
the thorax, bearing notochaetae and neurochaetae, and the
abdomen, with neurochaetae only. Branchiae occur dorsally
on segments 2 to 4; the filaments may be filiform, foliaceous, rosette-like in two or three groups
or may form a single large branchia bearing two to five
lamellate lobes. Ventral glandular pads are
absent. The pygidium is smooth or lobed and small cirri may
be present. Lateral organs and dorsal cirrus organs have not
been observed. There is a non-eversible ventral buccal organ
and the gut is looped. A gular membrane is present. Nephridia
are present as metanephridia; the relation to coelomostomes
has not been documented. The anteriormost pair of nephridia
are excretory in function; the posterior ones presumably act as
gonoducts. The circulatory system is closed and a heart body
is present. Aciculae are absent; other chaetae
are variously ornamented capillaries and uncini which may be
acicular, or avicular with short or long shafts.
The above description is taken from Hutchings (2000), which in turn is based on Fauchald & Rouse (1997).
Identification tips
Recognising the family
Trichobranchids are often confused with terebellids and ampharetids, although the branchiae are either single filaments arranged segmentally or composed of partially fused lamellate lobes, such branchiae and arrangement are not found in these other families. Unlike these other families thoracic neurochaetae are different to abdominal ones, they are long handled hooks whereas abdominal ones are avicular short handled uncini. Trichobranchids have a fixed number of thoracic segments, and a variable number of abdominal segments with neuropodia only. Trichobranchids tend to have short compact bodies and Artacamella has an eversible proboscis.
Distinguishing species
Characters used to distinguish species include the form and orientation of the chaetae, branchial structure, and the structure of the head (peristomium and prostomium).
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