5.5. Phylum Nemathelminthes: Roundworms

Roundworms, also called nematodes, have smooth, round bodies with pointed ends. Nematodes are so small that it is necessary to have a microscope to see them. Some others grow to 3 meters in length. They are found in huge numbers in soil, also roundworms are in freshwater and seawater, and in or on plants, humans, and other organisms. Roundworms are more complex than flatworms. The digestive system of a roundworm runs along the whole length of its body. The digestive system has two openings: the mouth and the anus. Many roundworms are parasites on animals. Others suck the juices from plant cells and cause them to wilt. But most roundworms are free-living in the soil. Some of them are helpful because they are decomposers. They digest dead and decaying matter and return materials to the soil. Many of these materials are useful to other living organisms. The ovaries and testes in most roundworms, as in most animals, are found in different individuals. During sexual reproduction, the mail roundworm transfers sperm to the female roundworm to fertilize her eggs. Roundworms make up the phylum Nematoda, so they are also called nematodes. Like flukes and tapeworms, most roundworms are parasites. Almost all species of plants and animals are affected by one of the 12,000 species of roundworms. One shovelful of garden soil may contain over 1 million nematodes. Roundworms feed on plants by sucking the juices from them. Growers of fruit trees, strawberries, vegetables, and cotton suffer annual financial losses due to roundworms. Humans are hosts to about 50 species of roundworms. Pinworms, hookworms, and intestinal roundworms are common human parasites. Nematodes have tubular bodies covered by a tough cuticle and tapered at both ends. A fluid-filled pseudocoelom provides a structure against which the worm’s longitudinal muscles can contract. The absence of circular muscles gives roundworms their characteristic thrashing whipping motion in water. The roundworm’s digestive system consists of a tube called the alimentary canal, which is open at both ends. The anterior opening is the mouth. The posterior opening is the anus, through which solid wastes leave the body. Liquid wastes are collected by a system of tubes and are expelled through an excretory pore in the worm’s posterior end.

Most roundworm species have separated male and female sexes. The females, which produce thousands of eggs, are usually larger than the males. The male guinea worm, for example, is about 2.5 cm long, while the female is 60 to 120 cm long. In the male reproductive system, sperm passes from the testes into the cloaca, a common chamber into which digestive, reproductive, and excretory systems empty. During mating the sperm leaves the cloaca and enters the female’s reproductive opening. Sperm is then stored in the female’s body and used to fertilize eggs when they mature. Young roundworms develop in the body of the female or, in some species, outside of the female’s body.

  But many of roundworms are free-living in the soil. Some of them are helpful because they are decomposers. They digest dead and decaying matter and return materials to the soil. Many of these materials are useful to other living organisms.

Ascaris

One of the largest nematodes that live in humans is the Ascaris, or intestinal roundworm. These worms grow to 30 cm in length. Ascaris also lives in the intestines of pigs and horses.The life cycle of Ascaris is typical of many roundworm species. The cycle begins when a human or other host eats vegetables grown in soil containing the eggs. The eggs hatch in the intestine of the host. The larvae bore through the intestinal wall, enter the bloodstream, and are carried to the lungs. The larvae are then coughed up into the mouth, swallowed and returned to the small intestine. There they develop into mature adults and reproduce.

Hookworms

The hookworm gets its name from its way of hooking onto the small intestine of its host. The worm feeds by sucking blood. This process can greatly reduce the number of red blood cells in the host’s bloodstream, causing severe anemia. Hookworms can also damage the host’s small intestine. Hookworm larvae develop in the soil and enter a human host through microscopic cracks in the soles of the feet. Like Ascaris, hookworms travel through the circulatory system to the lungs.