Flatworms, roundworms and annelids

Platyhelminthes (flatworms) have a body flattened from top to bottom (dorso-ventrally). Most are endoparasites of animals, humans included. They are bilateral, triploblastic, acoelomate and at the organ level.

Parasitic flatworms have hooks and suckers, and some take in food from the host straight through the body surface. Flame cells handle osmoregulation and excretion. They are hermaphrodite with internal fertilisation and many larval stages. Planaria regenerates very well. Examples: Taenia (tapeworm), Fasciola (liver fluke).

Aschelminthes (roundworms) are circular in cross-section. They may be free-living in water or soil, or parasites of plants and animals. They are bilateral, triploblastic, pseudocoelomate and at the organ-system level.

The roundworm alimentary canal is complete, and its pharynx is muscular and well developed. Wastes leave the body cavity through an excretory tube that opens at the excretory pore. Sexes are separate (dioecious) and females are often longer than males. Fertilisation is internal; development may be direct or indirect. Examples: the roundworm Ascaris, the hookworm Ancylostoma and the filaria worm Wuchereria.

Annelida live in water (marine and fresh) or on land, are free-living and sometimes parasitic. They are bilateral, triploblastic, coelomate, organ-system level and metamerically segmented; the name comes from the Latin annulus, a little ring.

Longitudinal and circular muscles move annelids. Nereis, an aquatic form, swims with lateral appendages called parapodia. Circulation is closed. Nephridia handle osmoregulation and excretion. Paired ganglia joined by lateral nerves to a double ventral nerve cord form the nervous system.

Earthworms and leeches are monoecious (both sexes in one animal), whereas Nereis is dioecious; all reproduce sexually. Examples: Pheretima (earthworm), Hirudinaria (a blood-sucking leech) and Nereis.