What animals?
As anterior means front and posterior means back, then most-if not all animals have a front and back, or an anterior and posterior. They are usually characterized by a mouth(anterior) or rectum/anus (posterior).
posterior
The kidneys have their anterior and posterior surfaces. The foremost surface countenances towards the front stomach wall, while the back posterior surfaces confronting the posterior stomach wall. These surfaces are isolated by the edges of the kidney, which are the significant convexity horizontally, and minor concavity medially.
Anterior, posterior and lateral also.
....behind/back and antonym would be anterior - in front/front
they have diagonal symmetry and they have bilateral.
Anterior
Not sure of the context, but: Anterior = front posterior = rear
If you refer to lungs in humans, then the lungs are anterior to the spine
the opposite of anterior is Posterior.
Which structure lies posterior to the bladder and anterior to the rectum?
Anterior has four surfaces while posterior has five.
It's relative to the subject. Anterior refers to the "beginning" while posterior refers to an "end"