Roundworms and Flatworms

This subcategory groups two separate worm phyla, the roundworms (Nematoda) and the flatworms (Platyhelminthes), both important across free living and parasitic lifestyles. Roundworms have a tube shaped, unsegmented body with a complete digestive tract running mouth to anus, and include free living soil and marine species as well as parasites of plants, animals, and humans such as Ascaris and Trichinella. Flatworms have a flattened, unsegmented body with no separate circulatory or respiratory system, ranging from free living planarians capable of regrowing a severed body from a small fragment to parasitic flukes and tapeworms that live inside a host's tissues or gut. Despite the different body plans, both phyla are studied together in parasitology for the diseases their parasitic members cause.