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.
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.
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.
Annelids are complex segmented worms. Earthworms, leeches, and many worms of the ocean make up the Annelid phylum. Annelids differ from the other worms in having a true coelom, giving them a “tube- within – a- tube” body plan. Most annelids are free-living. The leeches live as parasites. They attach themselves to animals in lakes and ponds and suck blood and other tissues from them.
The bodies of Annelids are arranged in segments, which can specialize in different tasks. Between segments 35 and 37 lies a swelling called the clitellum, which plays an important role in reproduction. The tissues of these complex worms are organized into many highly developed organ systems. Earthworms have pairs of bristles called setae.
Earthworms, which belong to the class Oligochaeta, live in soils all over the world. Earthworms vary in size from a few centimetres to 3.3 m long. A common North American species has a dark dorsal surface and light ventral surface. Most of the worm’s 100 to 150 segments are identical, except for the pointed anterior and posterior ends. Earthworms have pairs of bristles, called setae on each body segment except the first and last. An earthworm moves by anchoring the setae on its posterior segments and then contracting the circular muscles in front of the anchored segments. These contractions extend the body forward.
Digestion.An earthworm feeds by taking in soil with its muscular pharynx. The soil moves down a tube called the esophagus and then enters a storage chamber called the crop. From the crop, the soil moves to another chamber called the gizzard. Here, the grinding together of soil particles swallowed by the earthworm crushes pieces of organic matter. The food next moves into the intestine which extends to the posterior end of the worm. Folds in the wall of the intestine increase the surface area where absorption of digested food into the bloodstream takes place. Solid wastes pass out of the body through the anus.
Earthworms are very valuable to gardeners and farmers. As the worms eat their way through the soil, they break up soil clumps, aerate the soil, and add nutrients. Earthworms break down organic material faster than normal bacterial decomposition does.
Circulation. Unlike flatworms and roundworms, earthworms have a closed circulatory system. In a closed circulatory system, the blood circulates through a series of vessels. Dorsal and ventral blood vessels run through the length of the worm’s body. The blood absorbs molecules and carries them through the dorsal vessel to five pairs of muscular pumping tubes, or “heats”. These “heats” pump blood into the main ventral blood vessel. Smaller blood vessels carry the blood to all parts of the body.
Respiration and Excretion. Earthworms take in oxygen and give off carbon dioxide by diffusion through the skin. Because diffusion can occur only across a moist membrane, an earthworm must remain in an environment that is neither too wet nor too dry. Earthworms can drown in soil that is saturated with water because there is not enough oxygen. During dry periods, earthworms dig deep into the soil in search of moisture.
Earthworms eliminate liquid wastes through ciliated tubes called nephridia. The beating of cilia draws fluid from the coelom into a funnel-shaped opening in the nephridia. As the fluid passes through the tubules, needed water is reabsorbed by tiny blood vessels. Waste materials then pass out of the body through a pore in the skin. Each body segment has a pair of nephridia.
Nervous Control. An earthworms can respond rapidly to changes in its environment because of a concentration of nerve cells called a cerebral ganglion, near the worm’s anterior end. The cerebral ganglion is connected to the rest of the body by a ventral nerve cord that extends the entire length of the animal. A ganglion connects each body segment to the ventral nerve cord.
The earthworm has no external eyes or ears, but receptors in the skin which enable the worm to react to light, sound and chemicals.
Earthworms are active mainly at night and will move away from bright light. However, the light-sensitive cells of earthworms do not respond to red light. For this reason, earthworms to be used as finishing bait may be most easily dug up at night if red light is used for illumination.
Like planarians, earthworms are hermaphrodites. The female structures are located towards the anterior portion of the earthworm, and the male structures towards the posterior. Fertilization occurs when two worms exchange sperm. A mucous secretion from the clitellum holds the two earthworms together while they mate. The sperm each worm receives is stored in a seminal receptacle until just before the eggs are laid.
Two or three days after mating, the earthworm produces an external mucous case formed of sticky secretions from the clitellum. Muscular contractions push the case along the body. Mature eggs and sperm held in the seminal receptacle enter the case as it passes over the body of the worm. The case then seals forming a coat that protects the fertilised eggs until they hatch.
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