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Porifera, or sponges, have no true nervous system. Sponges live mainly in marine environments, and use water as their main means of existence. Sponges do not have brains, and also do not have functioning nerve cells, which means that sponges cannot feel, see, hear, or smell anything. They rely on water for their way of getting oxygen, removing waste, and digesting their food intake. With the sponge's absence of a nervous system, this characteristic makes them similar to plants. Porifera have a skeleton of spicules, which keeps their body structure rigid and sturdy. All sponges are filter feeders and have no internal organs. For much of their lives, sponges are sessile, meaning that they live attatched to an adult throughout their beginning stages of life.