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The ancient sponge appeared about 2.5 billion years ago—the first animal. Coming in many sizes and shapes, sponge bodies are a loose assemblage of cells held together by a special protein called collagen which is present in all animals. In addition, sponges have microscopic crystalline spicules that act as a skeleton. Sponge cells carry out all the functions that organs carry out in higher animals and they communicate with each other, an ability all animal cells share. Sponges are the only animals that if broken down to the level of their cells, they can reassemble themselves. Sponges also use their pumping to reproduce: they pump sperm and eggs into the water. They were the first animals to reproduce sexually.
