Worms – slimy and pretty gross, but they’re also pretty incredible if you think about it.
Worms are simple animals with elongated bodies, a mouth, gut, and anus. Unlike for advanced animals, they don’t have eyes, they don’t have brains, and they don’t have nerves. Most worms limbless, although, a number of species have many hair-like structures that help with movement.
Worms are invertebrate animals from a variety of distantly related groups. Worms include animals from phyla such as Annelida (earthworms, polychaetes), Nematoda (roundworms), Nemertea (ribbonworms) and Platyhelminthes (flatworms). To put this into perspective, mammals, birds, fish, and reptiles, all belong to the one phylum Chordata, i.e. a human is far more closely related to a fish than many worms are related to each other.
Where do worms live?
Worms are most common below ground, both on land and in aquatic environments. Earthworms, the most well-known worms, live within the soil all over the world. Polychaetes are almost absent on land and in freshwater but are massively abundant in marine habitats. Other worms live in a variety of habitats such as beneath the soil or sea floor, within a self-made tube, and within seaweed, plants and animals.
How do they reproduce?
There is a huge variety of ways that worms are able to reproduce. In polychaetes alone, there are at least 17 known methods of reproduction, including a range of both sexual and asexual methods. Sexual reproduction, which involves two individuals, is achieved with methods such as copulation (the method humans use), broadcast spawning (where eggs are released into water to be fertilized), and epitoky (a body segment filled with gametes that floats to the surface and then releases the eggs or sperm).
Worms are often very good at regenerating themselves and reproducing asexually. Many species are able to reproduce simply by splitting in half and producing a second identical individual, a process known as transverse fission. A second common method of asexual reproduction in worms is budding, where a worm grows a fully developed individual which then separates from its ‘mother’.
Body structure of worms
Worms have elongated bodies and are usually limbless. They have a very primitive head with just a mouth and simple sensory and nervous systems; no eyes, ears or nose. More ‘developed’ than jellyfish, corals and sponges; worms generally have three layers of tissue which allows for the formation of organs and a one-way digestive system so food can be moved through the body in one direction. In comparison, more primitive animals such as jellyfish and sponges take food in through the mouth, digest it, and then release the waste back out through their mouth.
- Most worms are less than 1 cm long but the world’s longest worms, ribbon worms, can reach massive lengths of over 50 m, making them the longest animals on Earth.
- A number of species of earthworms can grow longer the 60 cm (2 ft.) and the largest, the South African earthworms, can reach a massive length of over 6.5 m (22 ft.).
Last edited: 18 November 2018
FREE 6-Week Course
Enter your details to get access to our FREE 6-week introduction to biology email course.
Learn about animals, plants, evolution, the tree of life, ecology, cells, genetics, fields of biology and more.