There are thousands of nematodes. Not only are there more than 15,000 known species of roundworms, but there are many thousands of individual nematodes in even a single handful of garden soil. Some species of roundworm may contain more than 27 million eggs at one time and lay more than 200,000 of them in a single day. Some scientists have estimated that there may be as many as half a million more unknown species of roundworm yet to be discovered, an estimate based on the fact that many new species are still being discovered, that relatively few people are looking for more species, and that most roundworms are the second most diverse group of animals, trailing behind only the arthropods. Nematodes were once classified with a very large and heterogenous cluster of animals grouped together on the basis of their overall worm-like appearance, simple structure of an internal body cavity called a pseudocoelom, and the lack of features such as cilia and a well-defined head that are found in most animals.