- abalone
- abdomen
- Aborigines
- abscissa
- abundance
- abyssal
- abyssopelagic
- acraniate
- aculeate
- acuminate
- acute
- adaptation
- adult
- advected
- aggressive behaviour
- Agnatha
- agricultural
- algae
- allantois
- allele
- allopatric
- allozyme
- alveolus
- amnion
- amniote egg
- amorphous
- Amphibia
- amphipod
- anatomy
- anchialine
- andalusian
- angulate
- animal
- aniso-
- anisogamously
- annelids
- annuli
- Antarctic Convergence
- Antarctic Divergence
- Antarctica
- anterior
- Antilles
- anus
- apex
- aphotic
- appendage
- aquatic
- Arctica
- arcuate
- areola
- arid
- artery
- Arthropoda
- articulation
- association
- attenuate
- autotrophic
- axilla