- a-e seta
- A1
- A2
- Abaxial
- Abdomen
- abdominal
- Aboral
- Aboral pole
- abundance
- Acanthin
- acantho-
- accessory flagellum
- Accessory spines
- Achaetous
- Achaetous paddles
- Acicula
- acicular lobe
- Acicular setae
- Acraspedote
- actinost
- Actynopharynx
- aculeate
- acuminate
- acute
- adaptation
- Adaxial
- adductor muscle
- Adhesive disc
- adipose fin
- Adnate
- Adoral teeth
- Adoral zone of membranelles
- Adradial
- adult
- Aesthetasc
- aileron
- alga
- alimentary diverticulum
- Alveoli
- Anal cirrus
- anal fin
- anal tongue
- Anastomosing
- Anchor barbs
- animal
- Annular furrow
- Annular strictures
- Antarctic convergence
- Antarctic divergence
- Antecephalic chamber
- Antecephalic lobe
- Antenna
- antennal flagellum
- antennal spine
- Antennule
- anterior
- anterior fins
- anterior margin
- anterodorsal
- anteroventral
- Antitragus
- anus
- Apertural lip
- Apertural side
- Aperture
- aperture base
- aperture sinus
- Apex
- Apex angle
- apical
- Apical canal
- Apical horn
- Apical knob
- Apical pore
- Apical projection
- Apical spine
- Apical tube
- apical wing
- Apodous
- apomatus-chaetae
- apomorphic
- Apophyses
- Aposymbiotic
- Appendage
- Appendages
- appendix interna
- appendix masculina
- Arches
- Areolate
- Arm
- Arm crown
- Arm disc
- Arm-crown pillar
- arm-crown stalk
- armcrown
- Armor shell
- Arms
- Article
- ascidiozooid
- Asetigerous
- Aspinal caveola
- Aspinal furrow
- Aspinal pores
- Astral septum
- Astropyle
- asymmetrical gland
- Athecata
- athorybia stage
- Atlantic Ocean
- atoke
- atrial chamber
- atrial muscles
- atrium
- attenuated
- Aurophore
- Axial
- Axial rod
- Axial spine
- Axoneme
- Axoplast
- Axopod