Taken primarily from Kozloff, 1987, 1996 p. 137 (Copyright 1987, 1996, University of Washington Press. Used in this web page by permission of University of Washington Press)
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| Lower taxonomic level | Main Page | Alphabetic Index | Systematic Index | Glossary |
| 1a | With 12 pairs of elytra | Lepidonotus squamatus |
| 1b | With at least 15 pairs (rarely 13 or 14 pairs) of elytra | 2 |
| 2a | With 15 pairs (rarely 13 or 14 pairs) of elytra | 3 |
| 2b | With at least 18 pairs of elytra | 23 |
| 3a | With more than 50 segments; elytra restricted to the anterior and middle regions of the body, at least 18 of the posterior segments not covered by them | 4 |
| 3b | With fewer than 50 segments; elytra covering nearly all segments | 6 |
| 4a | ||
| 4b | ||
| 5a | ||
| 5b | ||
| 6a | At least some of the neurosetae bifid at the tip (the 2 teeth are not equal, however) | 7 |
| 6b | None of the neurosetaebifid at the tip | 12 |
| 7a | Notosetae thicker than the neurosetae | 8 |
| 7b | Notosetae about the same thickness as the neurosetae | Tenonia priops |
| 8a | Anterior pair of eyes (on the dorsolateral surfaces of the prostomium) not visible from the dorsal side (they are covered by outgrowths of the prostomium)(photo)(free-living in various habitats, and also commensal with other polychaetes) | Harmothoe imbricata |
| 8b | Anterior pair of eyes visible from the dorsal side (not covered by outgrowths of the prostomium) | 9 |
| 9a | ||
| 9b | ||
| 10a | ||
| 10b | ||
| 11a | ||
| 11b | ||
| 12a | ||
| 12b | ||
| 13a | ||
| 13b | ||
| 14a | ||
| 14b | ||
| 15a | ||
| 15b | ||
| 16a | ||
| 16b | ||
| 17a | ||
| 17b | ||
| 18a | ||
| 18b | ||
| 19a | ||
| 19b | ||
| 20a | ||
| 20b | ||
| 21a | ||
| 21b | ||
| 22a | ||
| 22b | ||
| 23a | With 18 pairs (rarely 19 pairs) of elytra (free-living or commensal, usually with other polychaetes) | Halosydna brevisetosa |
| 23b | With more than 18 pairs of elytra | 24 |
| 24a | Elytra on every segment posterior to segment 38; dorsal nuchal fold prominent and covering part of the prostomium | Hololepida magna |
| 24b | Elytra not on every segment posterior to segment 38; without a prominent nuchal fold | 25 |
| 25a | Lateral prostomial antennae inserted on the edge of the prostomium; notopodia abortive, with an acicula but without other setae | 26 |
| 25b | Lateral prostomial antennae inserted slightly ventral to the edge of the prostomium; notosetae present, though not necessarily on all notopodia, and sometimes few in number | 27 |
| 26a | ||
| 26b | ||
| 27a | Elytra on segments 2, 4, 5, then every other segment to 23, 26, 30, 32, 35; following segment 35, the arrangement of elytra is irregular, because they are not in sequence and not always in pairs; notosetae numerous, with pointed tips (commensal with the terebellid polychaetes Thelepus crispusand Neoamphitrite robusta, perhaps others) | Polyeunoa tuta |
| 27b | Elytra on segments 2, 4, 5, then every other segment to 23, 26, 29, then every other segment to the end of the body; relatively few segments near the posterior end have unpaired elytra; notosetae few, with rather blunt, indistinctly bifid tips, or absent | 28 |
| 28a | Margins of elytra extensively ruffled or folded (commensal with various sea stars) | Arctonoe fragilis |
| 28b | Margins of elytra smooth or only slightly wavy | 29 |
| 29a | Neurosetae (except on segment 2) of 2 types: those above the acicula slender and with blunt, bifid tips, those below the acicula stouter and with falcate, pointed tips; with a band of dark pigment extending across segments 7 and 8 (commensal with a wide variety of invertebrates, including sea stars, terebellid polychaetes, gastropods [especially Diodora aspera], and Cryptochiton stelleri) | Arctonoe vittata |
| 29b | Neurosetae essentially of 1 type: falcate and with pointed tips; without a dark band of pigment extending across segments 7 and 8 (commensal with a variety of invertebrates, including sea stars, the sea cucumber Parastichopus californicus, Cryptochiton stelleri, and terebellid polychaetes) | Arctonoe pulchra |
Arctonoe fragilis
Arctonoe pulchra
Arctonoe vittata
Harmothoe imbricata
Lepidonotus squamatus