The phylum Nematoda are commonly called roundworms and have unsegmented bodies and a layer of cuticle that is secreted by the hypodermis underneath the skin. Nematodes lack a coelum and instead have a small, fluid-filled cavity containing their reproductive organs and intestine.
Roundworms are skinny worms tapered at both ends. Most of the 12,000 identified species look very much alike and are hard to distinguish. They live almost everywhere including freshwater and saltwater and can be herbivores, carnivores or omnivores. The help the environment by aiding in decomposition by breaking down organic matter.
Nematodes breath using the entire surface of their skin; therefore, they lack a respiratory system. Most species have separate sexes while others are hermaphrodites. All species have a mouth and anus, giving them a complete digestive system. They all have teeth which they use to eat, and a pharynx which pumps food into the intestine. They lack a circulation system and nutrients are sent throughout the body via diffusion. Some species have simple eyes while others use light-sensitive organs disbursed throughout the entire body.
Nematodes include hookworms, heartworms and pinworms. Some of the more dangerous types of nematodes include the worm responsible for trichinosis in animals. This nematode is called the trichinella spiralis.