Identification key to all families from the Tropical Eastern Pacific (modified from Cutler 1994)

1. Tentacles encircling the mouth (peripheral tentacles); hooks, when present, are scattered......2

– Tentacles encircling the nuchal organ (nuchal tentacles), peripheral tentacles absent; hooks, if present, arranged in rings....................................................................................................................4

2. Longitudinal muscles of body wall in uniform continuous layer ................................... Golfingiidae

– Longitudinal muscles of body wall gathered into separate or anastomosing bands.........................3

3. Longitudinal and circular musculature with anastomosis; spindle muscle attached to the posterior trunk ........................................................................................................................ Siphonosomatidae

– Longitudinal and circular musculature in continuous bands; spindle muscle not attached to the posterior trunk ................................................................................................................. Sipunculidae

4. Hooks absent; contractile vessel with villi .......................................................................................... ............................................. Antillesomatidae ( Antillesoma mexicanum Silva-Morales et al., 2019 1)

– Hooks present; contractile vessel without villi...............................................................................5

5. Anal shield present ................................................................................................. Aspidosiphonidae

– Anal shield absent ................................................................................................. Phascolosomatidae

1 This species is separate from Antillesoma antillarum (Grube, 1859) because of differences in the length of the organisms and for the molecular divergences noted by Silva-Morales et al. (2019).