Invertebrates
Roundworm
Nematoda · also called nematodes
In a single handful of ordinary garden soil, thousands of roundworms go about their lives entirely unnoticed, part of a group so numerous and so poorly catalogued that its true diversity likely dwarfs almost every other kind of animal on the planet.
Overview
The roundworm, phylum Nematoda, is a simple, tube-shaped animal found across nearly every habitat on Earth, from ordinary soil to deep ocean sediment and polar ice. Its staggering diversity, spanning both free-living and parasitic lifestyles, combined with its outsized importance as a scientific model organism, make it one of the most consequential yet least visually appreciated animal groups known.
Small, simple, and everywhere
Few animal groups manage to be simultaneously this abundant, this poorly known, and this scientifically important, with a single roundworm species mapped down to the individual cell while the vast majority of its relatives remain entirely undescribed.
Status
Not Evaluated — an enormously diverse group found across nearly every habitat on Earth, not considered to be of conservation concern overall.
Roundworm: common questions
Are roundworms really one of the most common animals on Earth?
Yes, roundworms are believed to be among the most numerous animals on the planet, with scientific estimates suggesting hundreds of thousands to well over a million species, the vast majority still undescribed, found in habitats ranging from ordinary garden soil to deep ocean sediment, polar ice, and hot springs.
Why is a roundworm called Caenorhabditis elegans so important in science?
Caenorhabditis elegans, a small free-living roundworm species, is one of the most important model organisms in biological research, notable as the first animal to have its complete genome sequenced and, remarkably, the developmental fate of every single one of its cells mapped in detail, making it a foundational tool for studying genetics, development, and ageing.
Are all roundworms parasites?
No, while numerous roundworm species are parasites of plants, animals, or people, many other species live entirely freely in soil, water, or decaying organic material, feeding on bacteria, fungi, and other small organisms, making the group ecologically diverse rather than uniformly parasitic.
How is a roundworm's digestive system different from a tapeworm's?
A roundworm has a simple but complete digestive tract running from mouth to anus, allowing food to be processed and waste expelled separately, unlike a tapeworm, which has no mouth or digestive system at all and instead absorbs already-digested nutrients directly through its skin from within a host's intestine.