The defining feature of the Ecdysozoa is a collagenous/chitinous cuticle that covers the body, and the necessity to molt the cuticle periodically during growth. Nematodes are roundworms, with a pseudocoel body cavity. They have a complete digestive system, a differentiated nervous system, and a rudimentory excretory system. The phylum includes free-living species like Caenorhabditis elegans as well as many species of endoparasitic organisms such as Ascaris spp. They include dioeceous as well as hermaphroditic species. Embryonic development proceeds via several larval stages, and most adults have a fixed number of cells.
The tardigrades, sometimes called "water bears," are a widespread group of tiny animals with a segmented cuticle covering the epidermis and four pairs of clawed legs. Like the nematodes, they are pseudocoelomates and have a fixed number of cells as adults. Specialized proteins enable them to enter cryptobiosis, a kind of suspended animation during which they can resist a number of adverse environmental conditions.