Nematodes are unsegmented pseudocoelomate worms inconspicuous until they capture our attention by infesting us, our plants, or our animals. Because nematodes are round in cross section, they are also called roundworms. Their body cavity is a pseudocoel, defined as a space between embryonic endoderm and ectoderm; the pseudocoel lacks a peritoneum - the mesodermal lining of the coelomic body cavity. Nematodes are probably the most abundant animals living on Earth. About 80,000 species of nematodes have been described in the scientific literature; researchers estimate that nearly 1 million living species exist. These worms range from only 0.1 mm (100 µm) to about 9 m in length. The female giant nematode Dioctophyme renale is 1 m in length; the male is only half as long. Free-living nematodes are slender and cylindrical, tapering at both ends, typically about 1 mm in length. Parasitic nematodes have a variety of shapes, many saclike; the longest is a 9 m long parasite from a sperm whale.