Simple animals? Okay, first, there are no simple animals. They are extreme in their own way, you just cant see them. Like dogs, they have better hearing then us humans and can see in the dark. And Bees, they dont see the world like us they see it in ultra-violt.
And cats. They can squeez into a hole the size of your palm.
So dont ask what is the simpliest animal, because we humans are kind of below the talents animals have.
They are, indeed, the most simplest multi-cellular marine animal
ameoba
ProtozoaProtozoaProtozoaProtozoa
Cnidarians
The sponge.
any animal with a spine or similar. the simplest being known as a notochord. it can also be said that any animal descendant from Haikouichthys is a vertebrate.
ribbon worm
Organism.
The sponge is often considered the simplest multicellular macroscopic animal. Sponges lack tissues and organs, and their body is made up of specialized cells performing different functions. They are filter feeders that obtain nutrients from water passing through their porous bodies.
The simplest form of animal life is found in single-celled organisms, such as protists or bacteria. These organisms have basic structures and functions, without specialized tissues or organs. They rely on diffusion for nutrient exchange and do not exhibit complex behaviors like more advanced animals.
That is a very vague question. The simplest answer would be birth from an egg or uterus.
Evolution of brain is called cephalization. Worms are found to have the simplest brain mechanism and have simple straight forward nervous system i-e head to tail.