Biologists are not completely sure how many different species live on the Earth. Estimates of how many species exist on the Earth range from 1.5 million as high as 100 million. To this day, about 1.5 million species have been classified, but there are many more species that are not classified and are believed to exist. Most of the unclassified species includes insects, mollusks, sponges, worms, starfish, urchins, and crustaceans. These species are often difficult to find and identify because of their small size and the fact that they live in habitats that are difficult to explore, like the tropical rain forests, and the ocean.
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