Worms vary in size from microscopic to over 1 metre (3.3 ft) in length for marine polychaete worms (bristle worms);[1] 6.7 metres (22 ft) for the African giant earthworm, Microchaetus rappi;[2] and 58 metres (190 ft) for the marine nemertean worm (bootlace worm), Lineus longissimus.[3] Various types of worm occupy a small variety of parasitic niches, living inside the bodies of other animals. Free-living worm species do not live on land but instead live in marine or freshwater environments or underground by burrowing.

In biology, "worm" refers to an obsolete taxon, vermes, used by Carolus Linnaeus and Jean-Baptiste Lamarck for all non-arthropod invertebrate animals, now seen to be paraphyletic. The name stems from the Old English word wyrm. Most animals called "worms" are invertebrates, but the term is also used for the amphibian caecilians and the slowworm Anguis, a legless burrowing lizard. Invertebrate animals commonly called "worms" include annelids (earthworms and marine polychaete or bristle worms), nematodes (roundworms), platyhelminthes (flatworms), marine nemertean worms ("bootlace worms"), marine Chaetognatha (arrow worms), priapulid worms, and insect larvae such as grubs and maggots.

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In taxonomy, "worm" refers to an obsolete grouping, Vermes, used by Carl Linnaeus and Jean-Baptiste Lamarck for all non-arthropod invertebrate animals, now seen to be polyphyletic. In 1758, Linnaeus created the first hierarchical classification in his Systema Naturae.[4] In his original scheme, the animals were one of three kingdoms, divided into the classes of Vermes, Insecta, Pisces, Amphibia, Aves, and Mammalia. Since then the last four have all been subsumed into a single phylum, the Chordata, while his Insecta (which included the crustaceans and arachnids) and Vermes have been renamed or broken up. The process was begun in 1793 by Lamarck, who called the Vermes une espce de chaos (a sort of chaos)[a] and split the group into three new phyla, worms, echinoderms, and polyps (which contained corals and jellyfish). By 1809, in his Philosophie Zoologique, Lamarck had created 9 phyla apart from vertebrates (where he still had 4 phyla: mammals, birds, reptiles, and fish) and molluscs, namely cirripedes, annelids, crustaceans, arachnids, insects, worms, radiates, polyps, and infusorians.[6] Chordates are remarkably wormlike by ancestry.[7]

In the 13th century, worms were recognized in Europe as part of the category of reptiles that consisted of a miscellany of egg-laying creatures, including "snakes, various fantastic monsters, lizards, assorted amphibians", as recorded by Vincent of Beauvais in his Mirror of Nature.[8] In everyday language, the term worm is also applied to various other living forms such as larvae, insects, millipedes, centipedes, shipworms (teredo worms), or even some vertebrates (creatures with a backbone) such as blindworms and caecilians. Worms include several groups. The three main phyla are:

Familiar worms include the earthworms, members of phylum Annelida. Other invertebrate groups may be called worms, especially colloquially. In particular, many unrelated insect larvae are called "worms", such as the railroad worm, woodworm, glowworm, bloodworm, butterworm, inchworm, mealworm, silkworm, and woolly bear worm.

Worms may also be called helminths, particularly in medical terminology when referring to parasitic worms, especially the Nematoda (roundworms) and Cestoda (tapeworms). Hence, "helminthology" is the study of parasitic worms. When a human or an animal, such as a dog or horse, is said to "have worms", it means that it is infested with parasitic worms, typically roundworms or tapeworms. Deworming is a method to kill off the worms that have infected a human or animal by giving anthelmintic drugs.

Lobopodians are an informal grouping of extinct panarthropods from the Cambrian to the Carboniferous that are often called worms or "worm-like animals" despite having had legs in the form of stubby lobopods. Likewise, the extant Onychophora are sometimes called velvet worms despite possessing stubby legs.

Jumping worms, are non-native, invasive earthworms first confirmed in Wisconsin in 2013. Native to eastern Asia, they present challenges to homeowners, gardeners and forest managers. Jumping worms get their name from their behavior. When disturbed, they thrash, spring into the air and can even shed their tails to escape.

Endemic to parts of Asia, jumping worms (Amynthas spp.) first arrived in North America sometime in the late 19th century, probably in imported plants and other horticultural and agricultural materials. Since then, jumping worms have become widespread across much of the northeast, southeast and midwestern U.S. In 2013, jumping worms were confirmed for the first time in the upper Midwest, at the University of Wisconsin-Madison Arboretum.

Surprisingly, all earthworms in Wisconsin are non-native. There have been no native earthworms in Wisconsin since the last glacier moved through the state thousands of years ago, scouring the landscape down to the bedrock. The familiar earthworms we see in our gardens and on our fishing hooks originated in Europe, brought here by settlers. Although all earthworms can harm landscapes and forests, jumping worms may pose a bigger threat than European worms.

Jumping worms can quickly transform soil into dry, granular pellets with a texture like discarded coffee grounds. This altered soil structure is often unaccommodating to ornamental and garden plants and inhospitable to many native plant species. In addition, they can deplete the soil of nutrients, impact soil organisms, and in many cases, invasive plants thrive where jumping worms live.

Unlike most other kinds of earthworms, jumping worms are parthenogenic - they self-fertilize and do not need mates to reproduce. Each new generation begins with the production of hardened egg capsules, known as cocoons, that overwinter in the soil to hatch the following spring. Jumping worm cocoons are resistant to cold and drought and are as tiny as mustard seeds. Since they greatly resemble small bits of dirt, they are hard to see and so are often unknowingly moved in soil, mulch, potted plants, etc.

All earthworms, not just jumping worms, can harm forests by changing the soil structure and forest floor vegetation. Their feeding can result in a loss of soil moisture, compacted soil, exposed roots, erosion and an increase of pathogens and non-native plants. The result is less diversity of native plants and animals in delicate forest ecosystems.

Unwelcome guests: Beware the emergence of dreaded jumping worms [exit DNR] by Kathy Stahl, co-chair of the Lower Chippewa Invasives Partnership. A well-written article first published in the Dunn county News on July 7, 2018.

Worms are invertebrate animals with bilateral symmetry. Worms have a definite anterior (head) end and a posterior (tail) end. The ventral surface of worms and other organisms is the bottom side of the body, often closest to the ground. The dorsal surface is located on the upper part of the body facing the sky. The lateral surfaces are found on the left and right sides of the body. Figure 3.35 compares bilateral symmetry in a whale shark and a swimming plychaete worm. Organs for sensing light, touch, and smell are concentrated in the heads of worms. They can detect the kinds of environment they encounter by moving in the anterior direction.

Flatworms are more complex than cnidarians. Cnidarians have two layers of cells, the ectoderm and the endoderm; flatworms have a middle layer called the mesoderm between the other two layers (Fig. 3.16). This extra layer is important because its cells specialize into a muscular system that enables an animal to move around. Beginning with the flatworms, all the animals we will subsequently study have a mesoderm and muscular system. The cells of the ectoderm and endoderm are also more organized than similar cells of cnidarians. For the first time, we see groups of tissues that have evolved to form organs, such as the ones in the digestive, nervous, and excretory systems.

Like the cnidarians, flatworms have a digestive system with only a single opening into the digestive cavity, but in independently living marine flatworms the cavity branches into all parts of the body (Fig. 3.37 B). These flatworms feed through a pharynx. A pharynx is a long, tubular mouthpart that extends from the body, surrounds the food, and tears it into very fine pieces (Fig. 3.37 C and D). Cells lining the digestive cavity finish digesting the food. Then the dissolved nutrients move to other cells of the body. Undigested food passes back out through the mouth, as in the cnidarians. Parasitic tapeworms usually absorb their nutrients directly from the host, while parasitic flukes have retained a digestive system.

The excretory system removes waste products and excess water from tissues of flatworms. Flatworms have a surprisingly elaborate system to rid the body of wastes (Fig. 3.39). This network runs the length of the animal on each side and opens to the outside through small pores in the posterior region of the body. Connected to the tubes are tiny cells that move wastes and water from the tissues into the tubes. These cells contain flagella that beat back and forth, creating a current of fluid that constantly moves toward the excretory pores. Under a microscope the flagellar movement looks like a flickering fire, and the structure is called a flame bulb.

Species in the phylum Nematoda (from the Greek root word nema meaning thread) are better known as the roundworms (Fig. 3.41). There are about 25,000 species of nematodes formally described by scientists. Nematodes are found in almost every habitat on Earth. One species was first discovered living inside felt beer coasters in German alehouses. Studies of farmlands have found as many as 10,000 nematodes in 100 cubic centimeters (cm3) of soil. Nematodes are similarly abundant in marine and freshwater sediments where they serve as important predators, decomposers, and prey for other species like crabs and snails. 3df8ca78c1