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Phylum Nematoda is an abundant group of roundworms with about 30,000 identified species with an estimated total of over 500,000 unidentified species. These organisms posses an elongated, round, unsegmented body covered by a tough outer cuticle in which provides support and protection against harsh conditions. Nematodes have a hydrostatic skeleton, meaning they rely on fluid pressure as opposed to a hard skeleton due to their internal fluid-filled cavity, known as the pseudocoelom, that pushes against the muscles in the body wall.
Among the most evolutionarily successful groups of animals, roundworms can be found in virtually every ecosystem on earth from marine and freshwater to terrestrial. Some species are free-living while many others are parasitic relying on their hosts to complete their life cycle. Free-living nematodes often prey on other small animals such as protists and other worms, however some are adapted to eating algae or decaying organic matter making them important to benthic microbial food webs.
Defining Characteristics
Defining Characteristics
Cylindrical shape
Bilateral symmetry
Most microscopic, one species grows up to 8m in length
Cephalization: defined head region with sensory organs
Many are parasitic and rely on a host to complete their life cycle
Triploblastic body plan: endoderm, mesoderm, ectoderm
Blastocoelom: body cavity filled with fluid but not lined with peritoneum
Lack a true circulatory or respiratory system
Rely on diffusion for gas exchange and nutrient transport
Complete digestive tract: separate openings for mouth and anus that allows for maximized digestive efficiency
Simple nervous system
nerve ring
"sensory pits" called amphids used to detect external stimuli
Estimated 500,000 to one million undocumented species
Reproductive adaptations
Reproductive adaptations
Egg stage: Life cycle begins as a fertilized egg, often laid inside the host (for parasitic species)
Juvenile larval stage: Nematodes emerge from their egg as larva and typically undergo at least 4 molts before becoming an adult
Achieve growth through cell enlargement, must molt to create more room
Juvenile nematodes have same number of cells as adult nematodes
Adult stage: With separate males and females adults reproduce sexually to produce more eggs and restart the life cycle
Undergo internal fertilization (dioecious)
Where are they found?
Where are they found?
Almost every ecosystem across the planet with a wide range of requirements
Marine, freshwater, terrestrial, and parasitic environments
Free-living nematodes primarily inhabit intertidal spaces among bottom sediment particles.
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