Ecdysozoa: Nematoida

An unidentified free-living nematode found in marine sediments. The simplicity of a hydrostatic skeleton is evident Image and caption from Reefkeeping and Sharon Taxonomy

Of all the metazoan clades to have evolved since the Cambrian explosion, three in particular stand out as extraordinary for their abundance and diversity (each with a million or more species). These are the nematodes, the mites (acari) and the insects. Interestingly, all three are ecdysozoans, and two of the three are mostly microscopic, whilst the third includes a majority of forms built to a tiny scale. But of these three, the nematodes win out as the most numerous animals on Earth, being found in all environments, both free living and parasitic, and even deep under the Earth. Closely related to the nematodes.are the nematomorpha, a small phylum of parasitic worms. They mostly infect terrestrial insects and other arthropods, but reproduce in aquatic environments. Around 300 species are known. MAK120416