a) most are free living, but many are also parasitic

b) they are a group of many paradoxes:

- may be the most diverse phylum but only ~15,000 named spp.
- may be the most numerous metazoans on earth but few have seen them (most are very tiny, only parasitic forms get large)
- although few have seen them, some are serious human parasites (hookworm, pinworm, trichinella, filarial worm that causes elephantiasis) and crop pests
- although few have seen them, one species (Caenorhabditis elegans) is one of the most intensively studied 'model' animals; the entire cell lineage of every cell in the adult body is known!

- cylindrical or spindle-shaped bodies surrounded by tough cuticle
- have a mouth (with a muscular pumping pharynx), straight gut and terminal anus
- body wall muscles are exclusively longitudinal (how do they move?)
- 4 nerve cords: dorsal and ventral (locomotory), 2 lateral (sensory)