Phylum Nematoda

• Name means thread
• Unsegmented worms containing a digestive tract with 2 openings called a mouth and an anus (complete digestive system)
• Bilateral symmetry
• Very numerous (>80000 species), found in almost every environment (most acquatic environments, moist soil, tissues of plants ex. leaves, body tissues or fluids of animals)
• Both free-living and parasitic species exist (most free-living species are carnivores)
• Body is covered with a thick, tough cuticle (Cuticle protects the nematode from being digested by host)
• Only have lengthwise muslces: only produces whip-like movements.
• Muscular pharynx sucks food through the mouth into the intenstine
• Intestine is only very thin (1 cell layer thick) which allows diffusion of nutrients to occur efficiently.
• full digestion and absorption occurs in the intestine (also called the gut)
• body cavity (pseudocoleom) makes them unique.
• parasitic species do not have gland cells (no digestive enzymes produced) not required because the food is digested by the host.
• undigested food and waste leave through the anus
• O2 and CO2 enter/exit by diffusion across the body wall.
• no internal transport, all nutrients, gases, and wastes are transported by diffusion
• cellular wastes diffuse into the intestine and exit the body with the undigested food. some species have excretory canals which may remove excess H2O or waste
• nervous system consists of a ring of nervous tissue around the pharynx which is connected to two nerve cords. [transmit sensory information and control muscles, can sense and locate prey or potential hosts (chemical receptors )]
• Long bands of longitudinal muscles extend the entire length of the body (have no cilia or flagella), contraction and relaxation is controlled by the simple nervous system, movement is whip-like and thrashing.
• Only reproduce sexually, most species have saparate sexes (very few are hermaphroditic), internal fertilization occurs inside the female, reproductive organs take up lots of space inside that animal.
• Ascaris: adult worms live in human intestines where sexual reproduction occurs, fertilized eggs leave primary host’s body in feces, eggs hatch in the intestines of a new host who comes in contact with the infected feces, larval worms burrow into the intestinal wall and enter the host’s blood, tiny larval worms travel through the blood to the lungs, worms climb into the throat where they are swallowed and carried back into the intestines as adults.
• Ecological roles: food for larger organisms, help aerate soil (improves water uptake and gas exchange), parasitic species cause many serious diseases
• Necator americanus (”The American Killer” or Hookworm): similar life cycle as the Ascaris except it feeds on blood, causes energy loss and poor development in children.
• Trichinella spiralis (Trichina Worm): obtained by eating raw/undercooked pork, adult worms usually do not cause much harm and may disappear over time, migration of larvae through the bloodstream can cause muscle pain, weakness, fever, or death.