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.
