Eunicidae and the closely related Onuphidae include errant, mobile predators and tubicolous (tube-dwelling) scavenging species, indicating the non-phylogenetic nature of the old classification of polychaetes into Errantia and Sedentaria. The buccal organ in these taxa is entirely ventral; when well-developed projects backwards under the oesophagus as a muscular sac. The mouth is terminal unlike the ventral placement seen in Amphinomidae.
The proboscis has protractor and retractor muscles attached to it. A jaw apparatus present; this consists of a dorsal series (maxillae) of which the larger, at least, are joined in the midline to the other side.Ventral jaws on the floor of proboscis are known as mandibles.The maxillae are hardened and specialized parts of the edges of lateral folds projecting into the cavity of proboscis. The maxillary apparatus is constantly renewed. The chief variation is in the dental armature although the extent of the musculature also varies. The length of the proboscis very variable and reflects the length of the maxillary carriers or basal plates to which the main muscles are attached.
Very elongated errant, burrowing and tubiculous polychaetes. Prostomium lacking appendages or with 1 to 3 antennae and usually a pair of palps that resemble antennae. Sometimes with a pair of tentacular cirri on segment 2. Parapodia uniramous, the notopodium being represented by a branchia (gill) and a dorsal cirrus. Sometimes the dorsal acicula is present (in addition to the ventral), giving the sesquiramous condition (i.e. one and half rami). Setae simple or composite. The proboscis, when well developed, extends backwards as a muscular sac under the oesophagus and armed with dorsal maxillae and ventral mandibles.
Prostomium with 3 antennae and a pair of palps that resemble antennae.. Notopodium again represented by dorsal cirrus and branchiae but often supported by acicula. Australonuphis. Australonuphis teres, one of the the Australian Beach Worms, is a carnivore and scavenger and one of the largest annelids, reaching more than a metre in length with hundreds of segments. The head and first few segments are of typical errant form but from about trunk segment 6 posteriorly the parapodia are dorsolateral and leave between them a middorsal channel along which a respiratory and cleansing water current flows, these segments remaining in the burrow in the sand when the anterior segments are protruded for feeding. Diopatra is an onuphid that lives in a tube composed of shell fragments cemented by a fibrous organic secretion. The dorsal cirri of the parapodia are digitate and the branchiae are spiral and enlarged relative to those of Australonuphis.
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