Taken primarily from Kozloff, 1987, 1996 p. 164 (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 | Operculum well developed, at the tip of a specialized, nearly mid-dorsal radiole that has no pinnules | 2 |
| 1b | Operculum absent, or consisting of a swelling at the tip of a relatively unmodified radiole that has pinnules | 5 |
| 2a | Operculum globular, with a brown, chitinous plate at its distal end | Pseudochitinopoma occidentalis |
| 2b | Operculum funnel-shaped | 3 |
| 3a | Operculum with 2 or 3 obvious protuberances just below the funnel | 4 |
| 3b | Stalk of operculum without protuberances just below the funnel, but it has a ringlike thickening in this position | Serpula vermicularis |
| 4a | Operculum with 2 protuberances just below the funnel; edge of the funnel rather irregular, due in part to the funnel being longer dorsally than ventrally and somewhat compressed laterally | Crucigera irregularis |
| 4b | Operculum with 3 protuberances just below the funnel; funnel opening almost perfectly circular, its edge regular | Crucigera zygophora |
| 5a | Operculum absent | 6 |
| 5b | Operculum present, globular, transparent | 7 |
| 6a | With only 6 radioles | Salmacina tribranchiata |
| 6b | With about 120 radioles | Protula pacifica |
| 7a | Abdominal setae sharply bent, with smooth blades | Apomatus geniculata |
| 7b | Abdominal setae not sharply bent, with serrated blades | Apomatus timmsii |