- acanthancora
- acantho
- accessory
- accretive
- acerate
- actine
- actinocyte
- agglutinating
- ala
- alveolate
- amorphous
- amphiaster
- amphiblastula
- amphidisc
- amphitriaene
- anastomosing
- anatriaene
- anchor
- anchorate
- angulate
- aniso
- anisochela
- anisotropic
- anisoxea
- anthaster
- anthosigma
- aphodal choanocyte chamber
- aphodus
- apopyle
- aquiferous system
- arborescent
- archaeocyte
- arcuate
- areolated
- asconoid
- aspidaster
- aster
- astrorhiza
- atrium
- auxiliary
- axial