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Orbiniidae: Scoloplos normalis

Orbiniids have a regionated body; thoracic segments bear lateral parapodia and abdominal segments have dorsal parapodia. The prostomium lacks appendages and precedes one or two achaetigerous annuli. The ventral pharynx is unarmed and when everted is either sac-like or branched. Chaetae may be capillary, furcate or different types of spines, but rarely hooks. Parapodia are oriented laterally in the anterior part of the body, but this changes to a dorsal orientation of the parapodia in the posterior part of the body. Parapodia are biramous, though anterior ones lack aciculae and do not project far from the body wall. Each notopodium consists of a single postchaetal lobe which varies in shape from short and triangular through cirriform to foliaceous. Posterior ones are supported by one to several aciculae and carry crenulate capillaries and, sometimes, furcate chaetae. Neuropodia are more variable along the body of an individual. Thoracic neuropodia are low, ranging in shape from digitate to mammiform, unlike those on the abdomen which are more elongate, may be flanged or notched basally and are supported by one to several acicula.

Neuropodia carry crenulate capillaries as well as spines, which may be hooded and are often arranged in conspicuous fascicles in the thoracic neuropodia; subuluncini occur in Protoaricia and Nainereis. Abdominal neuropodia may carry modified crenulate capillaries or flail-tipped chaetae (as in Orbinia hartmanae or strongly bent, crested hooks (in Proscoloplos and Pettibonella). Age-related gain and loss of spines has been reported in members of Scoloplos. In Phylo species, stout modified spines emerge from epithelial glandular pouches in posterior thoracic segments, and may function in defence.

Dorsal and ventral cirri are absent. Podal, subpodal and stomach papillae occur in many species and their distribution is a useful specific character. Also of specific importance is the presence and distribution of interramal cirri (also called lateral organs when reduced in size to a low mound) of posterior thoracic and abdominal chaetigers. Segmentally paired branchiae are present over most of the body in most orbiniids Usually they are conspicuous straplike or triangular processes with fimbriated or ciliated lateral margins; in some genera (Nainereis, Protoaricia) cilia continue on dorsal ridges between the bases of the paired branchiae or on the dorsal edge of the notopodial lobe (Scoloplos). The pygidium carries one to many pygidial cirri, or cirri may be absent.

The above description is based on Glasby (2000), which in turn is based on Fauchald & Rouse (1997).

Identification tips

Reasonably complete individuals are needed for species identification, and posterior thoracic parapodiae need to be mounted in order to see if neuropodial hooks are present in addition to the capillaries. They cannot be seen just under the dissecting microscope. Branchiae may commence as very small papillae before developing into regular sized branchiae.

Recognising the family
Orbiniids should be readily recognised by the distinct division of the body into regions, with anterior ("thoracic") segments bearing lateral parapodia and posterior ("abdominal") segments bearing dorsal parapodia.

Distinguishing species
The form of the prostomium (pointed or truncate), segment on which the first branchiae appear, the presence or absence of stomach papillae, as well as parapodial and chaetal morphology are principal characters used to distinguish species of Orbiniidae.


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