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Oweniidae
Description
The prostomium is fused to the peristomium. In Owenia, the
head is a low, lobed terminal structure; in other taxa, it is
rounded frontally, sometimes inflated and bilobed. The peristomium forms a complete ring behind the
prostomium. Antennae are absent. The prostomial palps are
lobed in Owenia, but may be a pair of grooved palps which
emerge dorsally, or are lacking.
Nuchal organs have not been observed. The
longitudinal muscles are grouped in bundles, and
segmentation is present. The first segment is similar to the
second segment; in these segments only notopodia are
present. Subsequent segments have both notopodia and
neuropodia. Notopodia are short, truncated cylinders; the
neuropodia are wide with a flattened torus.
There are no dorsal or ventral cirri, branchiae or epidermal
papillae. Pygidial cirri are usually absent; although some
species have multiple pygidial cirri. The buccal organ is
eversible and ventral. Gular
membranes are absent, and the gut is a straight tube.
Segmental organs, termed mixonephridia,
open individually on one or a few anterior segments. The
circulatory system is closed, and a heart body is lacking. Aciculae are absent.
Variously ornamented capillary chaetae are
present in the notopodia and very small, unhooded dentate
hooks in the neuropodia.
The above description is based on Hutchings (2000), which in turn is based on Fauchald & Rouse (1997).
Identification tips
Until recently one oweniid, Owenia fusiformis was regarded as having a cosmopolitan distribution, however on examination using SEM of the neuropodial uncini has revealed a complex of sibling species and these are also being confirmed using molecular techniques (Michel Bhaud and his co-workers, personal communication). Description of new Australian species in this complex is imminent.
Recognising the family
Members of the Oweniidae have characteristic tubes which are considerable longer than the animal and are composed of shell fragments sand grains which stacked on top of each other (however, species of Phoronis (Phylum Phoronida) in Australia construct very similar tubes).
Distinguishing species
Oweniid genera and species are generally distinguished on the basis of head structures, number of thoracic chaetigers, and shape and dentition of the uncini. Increasingly SEM is being used to distinguish species especially in the genus Owenia, and the structure and dentition of the neurochaetae varies between species. Oweniids often remain intact within their tubes and must be carefully removed for proper examination and identification.
Description | Identification
tips | Natural History | Diversity
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