Macro photo of sea sponge Though extremely plant-like in appearance, sponges are actually one of the most primitive animals in the sea. They belong to a group called porifera. Most of us are familiar with the dried colorless varieties that populate the kitchens and bathrooms of the world. But in the ocean, live sponges can be found in an infinite variety of colors and shapes. Most of them are relatively small, but some varieties can grow to over 6 feet in diameter. Sponges differ from all other marine invertebrates in that they have no true tissues or organs. Their structure is composed of simple aggregations of cells.