Auerbach’s Plexus
A network of nerve fibres and ganglia in the gut musculature bringing about peristalsis when stimulated by pressure of food in the gut.

Auricle (Atrium)
(1) One of the chambers of the heart receives blood from veins and passes it to ventricle (q.v.). Has muscular walls, but is not as powerful a pumping organ as the ventricle. A fish has a single auricle, bit in land living (tetrapod) vertebrates, which breath mainly or entirely by lungs, there are two auricles, one receiving oxygenated blood from lungs, other deoxygenated from the rest of the body. Most Mollusca have to auricles, one receiving blood from each side of the body.
(2) External ear.
(3) (Bot.) Small ear or claw-like appendages occurring one each side at the base of leaf-blades in certain plants.

Australian Region(Notogea)
Zoogeographical region consisting mainly of Australia and New Guinea.

Autochthonous
Of soil micro-organisms, those which unlike zymogenous ones, have metabolic activity and population more or less stable, unaffected by addition of organic matter.

Autoecious
Of rust fungi (order Uredinales of Basidiomycetes), having the different spore forms of the life cycle all produced on one host species, e.g. mint rust.

Autogamy
(1) A curious sexual process found in some Protozoa and in some Diatoms in which the nucleus of an individual divides into two parts which reunite.
(2) Term also used for self-fertilization in plants.