- abdominal
- aboral
- abundance
- acantho-
- acetabuliferous
- acidophyl
- aculeate
- acuminate
- acute
- adaptation
- adaptativity
- Adriatic Sea
- adult
- alga
- allele
- allopatric
- allosperm
- AMNAPE
- amorphous
- anatomy
- animal
- ANSP
- antarctic convergence
- antarctic divergence
- anterior
- Antilles
- anus
- aperture
- aperture base
- aperture sinus
- apex
- apex angle
- apomorphic
- aquatic
- aragonite
- aragonite compensation depth
- arcuate
- arm
- articulation
- Atlantic Ocean
- atrium
- attenuated
- autosperm