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There are about 12,000 described species worldwide.
  • There are probably many more unrecognized species.
  • Many of the same species that exist in Canada are found in other places all over the world.
Many free-living nematodes are predatory carnivores and may have teeth for feeding, or a long spear-like structure which they use to stab and suck the juices out of their prey.
  • Other features of the mouth area, including spines, jaws, and other structures arranged in symmetrical patterns.
  • Nematodes feed on organic matter, both dead and alive such as small animals including other worms, or on diatoms, algae, fungi and bacteria.
  • Some eat plants by piercing the stem or root and sucking out the contents.
Round Worm
Round worms inhabit virtually every habitat in the seas, freshwater, and on land, although some species have very specific habitats.
  • Nematodes generally live in the spaces between aquatic sediments or on the sediment surface.
  • Non-parasitic nematodes are adapted to 'swimming' along the bottoms of streams and lakes.
The lives of round worms are diverse and somewhat complex, lasting from two days to over a year.
  • Some are free-living (have the ability to live on their own and have mobility) their entire lives, while others are only free living as adults or juveniles.
  • Some are parasitic on invertebrates, vertebrates, or even plants (meaning that the are not free-living).
  • When food or oxygen supplies become inadequate, most nematodes have the ability to suspend their life processes completely (like being dead).
  • In this suspended state, they can survive extreme drought, heat, or cold, and then return to life when conditions improve.
  • As it grows, the nematode periodically sheds its tough outer layer (called a cuticle), replacing it with a new one four times throughout its lifetime.
Nematodes usually possess separate sexes and mating will occur between a male and a female of the species.
  • In most freshwater species the males deposit sperm into the female, the fertilization occurs in the body of the female.
  • A thick double-layered shell forms around each fertilized egg, and these are usually deposited in the environment, where development takes place.
Round worms can be both predator and prey to other round worms (yes, the are cannibals).
  • Other predators include invertebrates such as crayfish, flatworms, and nemertean worms.
  • Parasitic round worms feed off of other animals such as invertebrates or vertebrates (ie. fish) or on plant juices.
  • Because round worms often comprise a large portion of a fresh water habitat's biomass, they are very important in the food chain. 
  • When placed in a fluid environment and deprived of contact with solid objects, benthic nematodes thrash about rather inefficiently. Some actually do swim (but not very well), and some are able to crawl along using various cuticular spines, grooves, ridges, and glands to gain purchase on the substrate.
  • All freshwater nematodes secrete a sticky mucous from the tip of their hind end which anchors the worm in place whether it be on a rock or inside an intestin.
  • Eutely is a phenomenon found in a few organisms, including nematodes, wherein each member of a species has exactly the same number of cells. For example, males of the species Caeonorhabditis elegans have exactly 1031 cells, while females have 959 cells, almost half of which are designated to the nervous system.

Brusca, R. C, and Brusca, G. J., 2003. Invertebrates, 2nd ed. Sinauer Associates, Inc., Publishers, Massachusetts. Pages 351, 353, and 358.
Canada's Aquatic Environments (2002). Nematoda. Available here.
University of California Museum of Paleontology (2010). Introduction to the Nematoda. Available here.
Waterwatch South Australia (2004).  Sponges, Hydras and Worms. Available here.