| Name | Children | Media (from resource) | Traits |
|---|---|---|---|
| Abarenicola Wells 1959 | 1 | 0 | 0 |
| Abarenicola pacifica Healy & Wells 1959 | 0 | 0 | 2 |
| Aberrantidae | 0 | 0 | 8 |
| Abyssoninoe Orensanz 1990 | 1 | 0 | 5 |
| Abyssoninoe hibernica (McIntosh 1903) | 0 | 0 | 26 |
| Aciculata unplaced | 2 | 0 | 0 |
| Acoetidae | 1 | 0 | 6 |
| Acrocirridae | 2 | 0 | 6 |
| Acrocirrus Grube 1873 | 1 | 0 | 0 |
| Acrocirrus frontifilis (Grube 1860) | 0 | 0 | 2 |
| Aeolosomatidae | 0 | 0 | 6 |
| Aglaophamus Kinberg 1866 | 2 | 0 | 2 |
| Aglaophamus agilis (Langerhans 1880) | 0 | 0 | 2 |
| Aglaophamus circinata (Verrill ex Smith & Harger 1874) | 0 | 0 | 2 |
| Alciopidae | 2 | 0 | 0 |
