The term “worm” is commonly used to describe a wide range of invertebrates that, in many cases, are not closely related to each other. 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 illness; the one thing they all have in common is a long, thin and flexible body. In most cases they are limbless, but some insect larvae that have short legs are often described as worms. Among the animals that fall into this rather ill-defined and unscientific category are earthworms, nematodes, flatworms, various insect larvae, and various marine invertebrates.
There are about 2,700 different types of earthworms. As their name suggests, they live on land and are generally considered beneficial as their movements mix the soil, keeping it well aerated and porous. Earthworms eat various types of dead organic matter, such as fallen leaves and other plant parts, and excrete waste that helps provide living plants with nutrients. In some cases, however, they can be considered a pest, as they can remove leaf litter required by other, sometimes endangered, species.
A silkworm, a moth larva, with its cocoon.
Earthworms usually live in burrows in the ground, which can be temporary or permanent. Some types rarely come out of their dens. In areas with cold winters, animals stay warm by burrowing deep into the ground, returning to the surface in spring when the ground warms. They move using tiny bristles along their sides, controlled by muscles, and breathe by absorbing oxygen directly through their damp skin. Although they do not have eyes, they are sensitive to light and avoid it.
Some caterpillars are considered worm-like larvae.
Some types of earthworms can reach considerable size. The types most commonly found in the US, often called “nightcrawlers”, typically grow to just over a 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. Australia’s Giant Gippsland Earthworm grows up to nine feet (three meters) in length, and a 22-foot (6.7 m) specimen has been reported from South Africa.
Humans can catch tapeworms from eating undercooked meat.
There are just under 20,000 known species of nematode worms, but the actual number could be much higher, as many types have not been studied closely, due to their generally small size and diverse habitats. They are extremely numerous and are believed 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 (1 mm) in length, but some are much longer – a 26-foot (8m) specimen was reportedly found in a sperm whale.
A large number of nematodes are found in the soil. Some are considered pests as they eat plant roots, but some are predators and can be beneficial to humans by eating various invertebrate pests, including other nematodes. Many species are parasitic and almost all animal species, including humans, can potentially harbor a parasitic nematode. Roundworms and hookworms, which can infect domestic animals and humans, are two common examples. Some other nematode infections, such as trichinosis, can be very serious.
Flatworms include both predatory and parasitic species. The reason they are flat is that they have no circulatory system – oxygen and nutrients reach the cells by diffusing through the tissue, so all cells must be close to the surface to receive oxygen and close to the intestine to receive nutrients from food. The intestine may be branched to allow for the distribution of nutrients to all tissues.
Among the most studied types of non-parasitic flatworms are planarians, 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 complete new animal. They are found in fresh and salt water and in moist 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 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.
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.