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Cossuridae
Description
The prostomium is bluntly conical. The peristomium forms a
distinct ring; it is sometimes referred to as an achaetous
segment and has
been often confused with a segment. Anterior appendages are lacking. A pair of nuchal organs are
present as short dorso-lateral ciliated grooves
on the posterior prostomium. The longitudinal muscles are
grouped in bundles, and segmentation is
distinct. The first segment and appendages are
similar to subsequent ones; parapodia have similar rami
and capillary chaetae. The parapodial rami are
often nearly confluent in the first or first few segments. Both
notopodia and neuropodia are low, ridge-shaped or papillalike
structures; dorsal and ventral cirri are absent.
A single median branchia (often referred to as a median
tentacle) is present dorsally on one anterior chaetiger.
Epidermal papillae are lacking.
Three or more pygidial cirri are present in some taxa. Lateral organs and dorsal
cirrus organs have not been observed. Aciculae are lacking;
the chaetae are limbate, slender capillaries,
and in one genus, Cossurella, acicular spines are also present
on posterior segments.
The above description is taken from Hutchings (2000), which in turn is based on Fauchald & Rouse (1997).
Identification tips
Recognising the family
Cossurids are small thread like worms with pointed or slightly rounded prostomium characterised by a single median branchial filament which tends to be retained. The absence of prostomial appendages, and the single dorsal branchial filament arising from a median position on an anterior segment distinguish cossurids from other polychaetes.
Distinguishing species
The chaetiger at which the dorsal branchial filament arises, whether chaetiger 1 is uniramous or biramous, the number of thoracic chaetigers, and the type of thoracic chaetae present are among the characters used to distinguish species of Cossura.
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