Figure 23.22: Nematodes (A) These unsegmented animals live up to their common name of roundworm, being round and wormlike in appearance. Most species are parasites of other multicellular organisms, aquiring nutrition from their hosts. The diagram shows Trichinella spiralis, a parasitic nematode that causes trichinosis in humans. (B) This micrograph shows larvae of T. spiralis encysted in the muscle tissue of a host swine.