<P> Arthropods total about 1,113,000 described extant species, molluscs about 85,000 and chordates about 52,000 . </P> <P> Sponges are animals of the phylum Porifera (Modern Latin for bearing pores). They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through them, consisting of jelly - like mesohyl sandwiched between two thin layers of cells . They have unspecialized cells that can transform into other types and that often migrate between the main cell layers and the mesohyl in the process . Sponges do not have nervous, digestive or circulatory systems . Instead, most rely on maintaining a constant water flow through their bodies to obtain food and oxygen and to remove wastes . </P> <P> Sponges are similar to other animals in that they are multicellular, heterotrophic, lack cell walls and produce sperm cells . Unlike other animals, they lack true tissues and organs, and have no body symmetry . The shapes of their bodies are adapted for maximal efficiency of water flow through the central cavity, where it deposits nutrients, and leaves through a hole called the osculum . Many sponges have internal skeletons of spongin and / or spicules of calcium carbonate or silicon dioxide . All sponges are sessile aquatic animals . Although there are freshwater species, the great majority are marine (salt water) species, ranging from tidal zones to depths exceeding 8,800 m (5.5 mi). </P> <P> While most of the approximately 5,000--10,000 known species feed on bacteria and other food particles in the water, some host photosynthesizing micro-organisms as endosymbionts and these alliances often produce more food and oxygen than they consume . A few species of sponge that live in food - poor environments have become carnivores that prey mainly on small crustaceans . </P>

Approximately how many species of sponge live in fresh water environments
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