- abaxial
- abdomen
- aboral
- acantho-
- acanthosoma
- accessory flagellum
- achaetous
- acicula
- acicular lobe
- acraspedote
- actinula
- aculeate
- acuminate
- acute
- adductor muscle
- adradius
- adult
- aesthetasc
- algae
- alima
- alimentary diverticulum
- amphicoronate
- anal tongue
- antenna
- antennal flagellum
- antennal spine
- antennula
- antennular lappet
- anterior
- anthoathecate
- anthomedusa
- antizoea
- anus
- aperture
- aperture base
- aperture sinus
- apex
- apex angle
- apical canal
- apical projection or process
- apical wing
- apodous
- apomatus-chaetae
- appendix interna
- appendix masculina
- arm-crown stalk
- armcrown
- arms
- ascidiozooid
- asymmetrical gland
- athecate
- athorybia stage
- atoke
- atrial chamber
- atrial muscles
- atrium
- attenuated