CONCEPTS IN BIOLOGY
PART V. THE ORIGIN AND CLASSIFICATION OF LIFE
23. The Animal Kingdom
Members of the Nematoda are extremely common animals, often called roundworms or nematodes. They have a simple, unsegmented, elongated body structure that consists of an outer epidermis covered by a thick, flexible cuticle. The cuticle is shed periodically as the animal grows. Nematodes are triploblastic and have a pseudocoelom (figure 23.17). Their gut is continuous, with a mouth at one end and an anus at the other. Few animals are found in as many diverse habitats or in such numbers as the roundworms. They live in marine and freshwater habitats, moist soil, and many live within other organisms as parasites. Soil nematodes are extremely common.
FIGURE 23.17. Nematode Worms
Nematode worms have a simple structure but have a complete gut with a mouth and an anus. Their body cavity is a pseudocoelom.
Although most roundworms are free-living, many are economically important parasites. Some are parasitic on plants, whereas others infect animals; collectively, they do billions of dollars worth of damage to crops and livestock. Roundworm parasites of animals range from the relatively harmless human intestinal pinworms Enterobius, which may cause irritation but do no serious harm, to Dirofilaria, which can cause heartworm disease in dogs. If untreated, this infection can be fatal. Often, the amount of damage inflicted by roundworms is directly proportional to their number. For example, hookworms (figure 23.18) feed on the host’s blood. A slight infestation often results in mild anemia, but a heavy infestation of hookworms can result in mental or physical retardation in children because of the severe anemia it causes.
FIGURE 23.18. The Life Cycle of a Hookworm
Fertilized hookworm eggs pass out of the body in the feces and develop into larvae. In the soil, the larvae feed on bacteria. The larvae bore through the skin of humans and develop into adults. The adult hookworms live in the digestive system, where they suck blood from the host. The loss of blood can result in anemia, mental and physical retardation, and a loss of energy.
23.8. CONCEPT REVIEW
22. Describe the general body structure of a nematode.