Heterospio longissima Ehlers, 1874.
Diagnosis (modified from Borowski 1994)
Body elongated, linear, divided in three regions. Anterior region with 6̅8 short chaetigers; median region with greatly elongated segments; posterior region with few short, inflated segments. Posterior part of prostomium with a pair of lateral nuchal organs. Proboscis eversible, epithelial pouch. One pair of grooved peristomial palps, easily lost, leaving scars on lateral surfaces. Anterior region with 3–8 pairs of cirriform branchiae, usually very long, one pair per segment, arising above notopodia, beginning from chaetiger 2. Anterior region with biramous parapodia provided with simple capillaries, with or without acicular spines; elongated segments with chaetae usually forming a cincture near anterior margin; chaetae simple capillaries, may be accompanied by subuluncini, aristae chaetae, and acicular spines; posteriormost inflated chaetigers provided with strong acicular hooks.
Following Parapar et al. (2014), the terms “thorax” and “abdomen” are not used in the diagnosis due to the lack of a clear criterion about their limits. The presence of nuchal organs on the prostomium was included in the diagnosis. The range of 7̅9 short anterior chaetigers provided by Borowski (1994) is modified to 6̅8, following Parapar et al. (2014). Peristomial palps were qualified as “easily deciduous” because they are not reported in most descriptions. More details are provided about the number (range) and size of branchiae. The term “parapodia biramous” is used here only for anterior segments; it is also highlighted that in elongated segments there may be a change in the arrangement of chaetae, i.e., forming cinctures, although those seem not be present in H. longissima Ehlers, 1874 sensu stricto, H. reducta Laubier, Picard & Ramos, 1973 or H. angolana Bochert & Zettler, 2009.