Phylum Nematoda

The phylum Nematoda is made up of roundworms, worms with long, slender bodies that taper at both ends. Roundworms are among several phyla of animals known as pseudocoelomates. Pseudocoelomates are so named because they have a pseudo-coelom, which is a hollow, fluid-filled cavity that is lined by meso-derm on the outside and endoderm on the inside.

Roundworms range in length from less than 1 mm to 120 cm (4 ft). In contrast to cnidarians, ctenophores, and flatworms, which have a gastrovascular cavity with a single opening, round-worms have a digestive tract with two openings. Food enters the digestive tract through the mouth at the anterior end, and undigested material is eliminated from the anus (AY-nuhs) at the posterior end. A digestive tract represents a significant advancement over a gastrovascular cavity because food moves through the tract in only one direction. This allows different parts of the tract to be specialized for carrying out different functions, such as enzymatic digestion and absorption of nutrients. Most roundworms have separate sexes and are covered by a protective, noncellular layer called the cuticle (KYOO-ti-kuhl).

About 15,000 species of roundworms are known, but biologists estimate that there may be 500,000 or more species. The vast majority of roundworm species are free-living on land, in salt water, and in fresh water. One free-living roundworm, Caenorhabditis elegans, is a favorite organism of scientists studying developmental biology. However, more than l50 species of roundworms are parasites of plants and animals. These parasitic species damage plant crops and can harm livestock, pets, and humans. Humans are host to about 50 roundworm species. As you read about these roundworms, notice their adaptations for parasitism.

objectives

• Describe the body plan of a nematode.

• Outline the relationships between humans and parasitic roundworms.

• Describe the anatomy of a rotifer.

vocabulary roundworm cuticle hookworm trichinosis pinworm filarial worm elephantiasis rotifer mastax cloaca parthenogenesis

Garden—Just Add Water

Some garden supply companies now sell kits containing millions of microscopic roundworms. When released, these roundworms seek out and kill hundreds of varieties of insect pests, fleas, and ticks. Other types of soil-dwelling roundworms consume bacteria and fungi that attack plants. These roundworms are also known as beneficial nematodes.

However, not all roundworms are good for plants. Some species parasitize the roots of plants. Effective pest control with roundworms requires a knowledge of which species are harmful and which are beneficial.

figure 34-7

This pig intestine is completely blocked by roundworms of the genus Ascaris.

figure 34-7

This pig intestine is completely blocked by roundworms of the genus Ascaris.

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Ascaris

Ascaris (AS-kuh-ris) is a genus of roundworm parasites that live in the intestines of pigs, horses, and humans. Ascarids feed on the food that passes through the intestines of their host. As Figure 34-7 shows, they can become so numerous that they completely block the host's intestines if left untreated. The adult female can reach lengths of up to 30 cm (1 ft). The smaller male has a hooked posterior end that holds the female during mating.

One ascarid female can produce up to 200,000 eggs every day. The fertilized eggs leave the host's body in feces. If they are not exposed to sunlight or high temperatures, they can remain alive in the soil for years. Ascarid eggs enter the body of another host when the host ingests contaminated food or water. The eggs develop into larvae in the intestines. The larvae bore their way into the bloodstream and are carried to the lungs and throat, where they develop further. They are coughed up, swallowed, and returned to the intestines, where they mature and mate, completing the life cycle. If the infection is severe, larvae in the lungs can block air passages and cause bleeding from small blood vessels.

Hookworms

Hookworms are another group of intestinal parasites. As you can see in Figure 34-8, a hookworm's mouth has cutting plates that clamp onto the intestinal wall. Hookworms feed on their host's blood. Because they remove much more blood than they need for food, a heavy hookworm infection can cause anemia. Hookworm infections in children can result in slowed mental and physical development.

Like ascarids, hookworms release their eggs in the host's feces. The eggs produce larvae in warm, damp soil, and the larvae enter new hosts by boring through the host's feet. They then travel through the blood to the lungs and throat. Swallowing takes them to the intestines, where they develop into adults. Hookworms infect about one billion people worldwide. Most infections occur in tropical and subtropical regions.

figure 34-8

This SEM shows the hookworm Ancyclostoma duodenale, which uses its plates to cut into the host's intestine, releasing blood on which the hookworm feeds.

Trichinella

Roundworms of the genus Trichinella infect humans and a variety of other mammals, including pigs. Adult trichina worms live embedded in the walls of the host's intestine. They produce larvae that travel through the bloodstream to the muscles, where they form cysts. Figure 34-9 shows several such cysts. People become infected when they eat undercooked meat—usually pork—that is contaminated with cysts. After they are eaten, the cysts release the larvae, which burrow into the intestinal wall and mature into adults. Trichina infections are responsible for the disease trichinosis (TRlK-i-NOH-sis), which causes muscle pain and stiffness. It can even cause death if large numbers of cysts form in the heart muscle. However, trichinosis is now rare in the United States. Farmers no longer feed raw meat to hogs, and meatpackers generally freeze pork, which kills the worms. Thoroughly cooking pork also prevents trichinosis.

Other Parasitic Roundworms

The most common roundworm parasite of humans in the United States is the pinworm, Enterobius. School-age children have the highest infection rate, which in some areas is as many as 50 percent of children. Despite their high rate of infection, pinworms do not cause any serious disease. Adult pinworms are 5-10 mm (0.2-0.4 in.) in length and resemble white threads. They live and mate in the lower portion of the intestine. At night, the females migrate out of the intestine and lay eggs on the skin around the anus. When an infected person scratches during sleep, the eggs are picked up by the person's hands and spread to anything the person touches. Eggs that are ingested hatch in the intestine, where the worms develop into adults.

Filarial (fuh-LAR-ee-uhl) worms are disease-causing roundworms that infect over 250 million people in tropical countries. The most dangerous filarial worms live in the lymphatic system, a part of the circulatory system that collects excess fluid around cells and returns it to the blood. The adult worms can be as long as 100 mm (4 in.). The larvae they produce enter the blood and are picked up by mosquitoes that draw blood from an infected person. The larvae develop into an infective stage inside the mosquitoes and are injected into the blood of another person when the mosquitoes feed again. Inside the new host, the larvae complete their development and settle in the lymphatic system. When they are present in large numbers, filarial worms can block the lymphatic vessels, causing fluid to accumulate in the limbs. In severe cases, the limbs become extremely swollen and skin hardens and thickens, a condition known as elephantiasis (EL-uh-fuhn-TIE-uh-sis). Another type of filarial worm infects dogs and cats. Large numbers of Toxocara canis or T. cati live in the heart and large arteries of the lungs and are responsible for heartworm disease.

figure 34-9

Larvae of the trichina roundworm coil up inside cysts in their hosts' muscle tissue. The cysts are stained blue in this light micrograph (100x).

figure 34-9

Larvae of the trichina roundworm coil up inside cysts in their hosts' muscle tissue. The cysts are stained blue in this light micrograph (100x).

Comparing Flatworms and Roundworms

Materials living planarian, preserved specimens of tapeworms, male and female ascarids, hand lens or stereomicroscope Procedure Examine specimens of tapeworms, ascarids, and the living planarian. Try to locate the following structures on each worm: anterior end, posterior end, mouth, eyespot, hooks, suckers, and anus. Draw each worm and label each of the features you located.

Analysis

1. List the features found on the posterior and anterior end of each worm.

2. Which worm has a separate mouth and an anus?

3. Which worm "absorbs" digested nutrients?

4. Which worm has a digestive tract?

figure 34-10

Cilia surrounding the mouth of a rotifer sweep food into the animal's digestive tract.

P, figure 34-10

Cilia surrounding the mouth of a rotifer sweep food into the animal's digestive tract.

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