The term “worm” is commonly used to describe a wide range of invertebrates that are, in many cases, not closely related to one another. Some live in the soil, some in the sea, and some are parasites; some are beneficial to man, some are pests, and some can cause serious disease; the only thing they all have in common is a long, thin, flexible body. In most cases, they do not have limbs, but some insect larvae that do possess short legs are frequently described as worms. Among the animals that come under this rather ill defined and unscientific category are earthworms, nematodes, flatworms, various insect larvae, and a number of marine invertebrates.
Earthworms
There are around 2,700 different types of earthworm. As their name suggests, they live in the earth, and they are generally regarded as beneficial, as their movements mix the soil, keeping it well aerated and porous. Earthworms eat various types of dead organic material, such as fallen leaves and other plant parts, and excrete waste that helps supply living plants with nutrients. In some cases, however, they can be considered a pest, as they may remove leaf litter that is required by other, sometimes endangered, species.
Earthworms generally live in burrows in the soil, which may be temporary or permanent. Some types rarely leave their burrows. In areas with cold winters, the animals stay warm by burrowing deep into the soil, coming back up to the surface in spring when the ground warms up. They move using tiny bristles along their sides, controlled by muscles, and breathe by absorbing oxygen directly through their moist skins. Although they have no eyes, they are sensitive to light and will avoid it.
Some types of earthworm can grow to a considerable size. The types most commonly found in the USA, often called “nightcrawlers”, typically grow to a little over one foot (30 centimeters) in length, but the largest North American species, the endangered Giant Palouse worm, can reach three feet (one meter). Much larger types are found in other parts of the world. The Giant Gippsland Earthworm from Australia grows up to nine feet (three meters) in length, and a 22 ft (6.7 m) specimen was reported in South Africa.
Nematodes
There are just under 20,000 known species of nematode worms, but the true number may be much higher, as many types have not been studied closely, due to their usually small size and diverse habitats. They are extremely numerous, and are thought to be the most abundant animals on the planet — a small sample of soil will contain many thousands of them. The vast majority of species are very small, often less than 0.04 inches (1mm) long, but a few are much longer — a 26 foot (8m) specimen was reportedly found in a sperm whale.
Huge numbers of nematodes are found in soil. Some are considered pests, as they eat plant roots, but some are predatory and may be beneficial to man by eating various invertebrate pests, including other nematodes. Many species are parasitic, and just about every animal species, including humans, can potentially harbor a parasitic nematode. Roundworms and hookworms, which can infect domestic pets and humans, are two common examples. Some other nematode infections, such as trichinosis, can be very serious.
Flatworms
The flatworms include both predatory and parasitic species. The reason they are flat is that they have no circulatory system — oxygen and nutrients reach cells by diffusing through tissue, so the cells must all be near the surface to receive oxygen and near the gut to receive nutrients from food. The gut may be branched, to enable distribution of nutrients to all tissues.
Among the most studied types of non-parasitic flatworms are the planarians, which are best known for their ability to regenerate lost body parts. Planarians can be cut in half, or even into smaller pieces, and survive, with each part eventually growing into a new, complete animal. They are found in both fresh and salt water, and in damp soil.
Many other types of flatworm are parasites. Among the best known are tapeworms, which live in the intestines of mammals, absorbing pre-digested food. Some types can grow to over 65 ft (20 m) long in land mammals, and whale tapeworms reaching 100 ft (30 m) have been reported. In humans, these parasites are usually picked up from undercooked meat. Liver flukes, which often affect sheep, are another type of parasitic flatworm.
Wormlike Insect Larvae
Many insects have larvae that are commonly described as worms. For example, inchworms, — the caterpillars of geometrid moths — have three pairs of legs at the front of their bodies and two to three pairs at the back, and move with a looping motion. This, combined with the fact that many types grow to around an inch (2.54 cm) long, gives them their name: they look as if they are measuring out inches. There are around 1,200 species of geometrid moth in North America, and many more in other parts of the world.
One interesting type of inchworm is called the cankerworm. It can produce a thin line made of silk, similar to a spider web. The threads are often produced when the caterpillar has to drop from a tree in order to evade a predator. Cankerworms come in a variety of colors, but they all have distinctive long horizontal stripes on their bodies. They are one of the most destructive pests to crops, and often feed on fruit trees.
Marine Worms
The polychaetes, or bristle worms, are the most commonly seen marine worms. They have segmented bodies with prominent bristles, and many species live in burrows in sand or mud at the seashore or in shallow coastal waters, although some species are found on the sea floor under deep water or among coral reefs. Bristle worms sometimes cement together sand or grit particles to construct tubes, which they live in. They are mostly predators, but some species may scavenge. Some types are very brightly colored, and a few are luminous.