Classes Trematoda And Monogenea

The classes Trematoda and Monogenea consist of parasitic flukes, leaf-shaped flatworms that parasitize many kinds of animals, including humans. Some flukes are endoparasites that live in the blood, intestines, lungs, liver, or other organs. Others are ectoparasites that live on the external surface of aquatic hosts, such as fish and frogs. Trematodes tend to parasitize a wide range of hosts, whereas monogeneans are mostly ectoparasites of fish and other aquatic animals.

figure 34-3

Suckers on this blood fluke, Schistosoma mansoni, attach the fluke to the blood vessels of its host. (SEM 550x)

figure 34-3

Suckers on this blood fluke, Schistosoma mansoni, attach the fluke to the blood vessels of its host. (SEM 550x)

figure 34-4

In the life cycle of schistosomes, fertilized eggs are released into the host's blood vessels. The eggs pass out of the primary host in feces or urine. In water, the eggs develop into ciliated larvae. The larvae burrow into certain species of snails, which serve as intermediate hosts. The larvae develop tails, escape from the snail, and swim about. The tailed larvae bore through the exposed skin of a person and settle in his or her blood vessels. There, the larvae develop into adults, and the cycle repeats.

Structure of Flukes

A fluke clings to the tissues of its host by an anterior sucker and a ventral sucker, which are shown in Figure 34-3. The anterior sucker surrounds the fluke's mouth, which draws the host's body fluids into the gastrovascular cavity. A fluke's nervous system is similar to a planarian's, but flukes have no eyespots and their other sensory structures are very simple. The external surface of a fluke is covered by a layer called the tegument. The outer zone of the tegument consists of a layer of proteins and carbohydrates that makes the fluke resistant to the defenses of the host's immune system. The tegument also protects the fluke against the enzymes secreted by the host's digestive tract.

Reproduction and Life Cycle of Flukes

Most flukes have highly developed reproductive systems and are hermaphroditic. Fertilized eggs are stored in a fluke's uterus, which is a long, coiled tube, until they are ready to be released. Each fluke may release tens of thousands of eggs at a time.

Flukes have complicated life cycles that involve more than one host species. A good example is provided by the trematode blood flukes of the genus Schistosoma, as shown in Figure 34-4. Adult schistosomes live inside human blood vessels. Therefore, a human is the schistosome's primary host, the host from which the adult parasite gets its nourishment and in which sexual reproduction occurs. Unlike most flukes, schistosomes have separate sexes. Eggs produced by the female are fertilized by the male. In step O, some of the fertilized eggs make their way to the host's intestine or bladder and are excreted with the feces or urine. Human feces and urine often pollute freshwater supplies in regions with poor sewage control. In step ©, the eggs that enter fresh water develop into ciliated larvae that swim. In step G, if the larvae encounter a snail of a particular species, such as one of genus Oncomelania, within a few hours, they burrow into the snail's tissues and begin to reproduce asexually. The snail serves as the schistosome's intermediate host, the host from which the larvae derive their nourishment. O Eventually, the larvae develop tails and escape from the snail. G These tailed larvae swim through the water. If they find the bare skin of a human, they penetrate the skin, enter a blood vessel, and develop into adults. The cycle then begins again. Exposure to the larvae can happen when humans swim, bathe, wash clothes, or work in fresh water that contains the larvae.

Not all schistosome eggs leave the human body, however. Many are carried by the blood to the lungs, intestines, bladder, and liver, where they may block blood vessels and cause irritation, bleeding, and tissue decay. The eggs may also penetrate the walls of veins and the small intestine or urinary bladder, where they can cause much tissue damage and bleeding. The resulting disease, called schistosomiasis (SHis-tuh-soh-MIE-uh-suhs), can be fatal. It affects about 200 million people worldwide, mostly in Asia, Africa, and South America.

Other kinds of flukes cause less-serious diseases in humans. For example, a small brown fluke common in freshwater lakes of North America is responsible for swimmer's itch, a condition characterized by minor skin irritation and swelling.

Word Roots and Origins schistosomiasis from the Greek schizein, meaning

"to cleave or split," and soma, meaning "body," and iasis, meaning "process or condition"

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