Porifera (sponges) are considered to be the simplest organisms because they have no organs or organ systems.
Porifera is a phylum of the kingdom Animalia. They are aquatic organisms. They are invertebrates and are considered to be the simplest animals. Porifera stands for pore-bearing.
Some of the species of this phylum live in freshwater lakes and streams. The majority of the species live in marine habitats.
The adult Porifera are always sessile. This means that they always live as being attached to rocks or any other object. They are unable to move on their own.
Porifera do not have an organ level or organ system level of organization.
Therefore they are considered to be the simplest animals.
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