Earth Science: Biology – Animals – Worms
Worms include members of several different phyla, including flatworms (Platyhelminthes), roundworms (Nemathelminthes), and segmented worms (Annelida).
Most flatworms have a flat, sometimes paper-thin, body structure, filled with loose connective tissue. They lack advanced breathing organs and blood vessels and the digestive system ends blind without an anus. The nervous system spreads through the body like a mesh, with a
single large nerve center, or ganglion.
The reproductive organs may be quite complicated and in many species they take up most of the body cavity. Some flatworms live independently and others are parasitic. The most common flatworms include tapeworms, planarians, and leeches (see in focus).
Roundworms
Roundworms are mostly round and often threadlike without segmentation. A layer of muscle below the skin allows for snakelike movements. The body cavity is usually filled with fluid. The most commonly known roundworms are the nematodes, which differ from
flatworms by having a digestive tract with two openings: a mouth and an anus.
Their reproductive organs are less complex than those of flatworms. Aquatic, terrestrial, and parasitic lifestyles are all common. Some species of roundworms may even target people, such as the Ascaris lumbricoides, which can reach about 16 inches (40 cm) in length.
Humans can also be infested by any roundworms, such as the Trichinella spiralis, carried by their pets. Many roundworm species also target certain types of plants, such as potatoes, and can seriously damage crops.
Segmented worms
Segmented worms belong to the phylum Annelida. Their bodies are divided into segments, allowing them to have a closed circulatory system and a highly developed nervous system with nerve cords running the length of the body and branching out in each segment.
Many species have advanced eyes and sensory organs. Common annelids are earthworms {Lumbricusterrestris) and lugworms (Arenicola marina).
SCHISTOSOMIASIS
Schistosomiasis or bilharzia is a parasitic disease common in subtropical and tropical regions caused by flatworms of the genus Schistosoma. Its larvae enter the human body through the skin to mature.
Female adult schistosomes then lay eggs that leave the body via urine or feces. If infected people do not receive medical attention, chronic infections of the liver, intestines, and bladder will develop.
About 200 to 400 million people worldwide are estimated to suffer from this parasitic infection
BASICS
PARASITIC NEMATODES (round-worms) in whales may reach a length of up to about 30 feet (9 m).
GARDENS can be home to as many as 37 earthworms per square foot (400 per m2).