CONCEPTS IN BIOLOGY
PART V. THE ORIGIN AND CLASSIFICATION OF LIFE
23. The Animal Kingdom
The simplest bilaterally symmetrical animals are the flat- worms. Because they are also triploblastic, they represent a major increase in complexity over the sponges and cnidari- ans. They are considered more primitive than other bilaterally symmetrical, triploblastic animals because they lack a coelom and have only one opening to the gut. They have no circulatory or respiratory system. Their flat structure allows for the diffusion of gases between their environment and the internal cells.
There are three basic types of flatworms. Turbellaria is a class of free-living flatworms (often called planarians). The other two groups are parasitic: Trematoda (flukes) and Cestoda (tapeworms). These parasitic kinds constitute the majority of species of flatworms (figure 23.14). We may consider this form of nutrition rather unusual, but of all the kinds of animals in the world, more are parasites than not.
FIGURE 23.14. Flatworm Diversity
(a) Turbellarians are free-swimming, nonparasitic flatworms that inhabit marine and freshwater. (This is a marine species.) (b) Adult tapeworms are parasites found in the intestines of many carnivores. (c) Some flukes live attached to the outside surface of animals, but most are internal parasites of the lungs, liver, and intestine of vertebrate animals.
Turbellarians are free-living flatworms that are bottom dwellers in marine waters or freshwater. A few species are found in moist terrestrial habitats. They are carnivores or scavengers that feed on dead organisms. The food that enters the gut is partly broken down; then, the food particles are engulfed by the cells that line the gut. Many marine species are brightly colored.
Flukes are common parasites. Some flukes are external parasites on the gills and scales of fish, but most are internal parasites. Most flukes have a complex life cycle involving more than one host. Typically, the larval stage infects an invertebrate host (usually a mollusk), whereas the adult parasite infects a vertebrate host. They are commonly found in the gut, liver, or lungs of vertebrate hosts.
For example, the disease schistosomiasis is caused by adult Schistosoma mansoni flukes, which live in the blood vessels of the human digestive system. This disease is common in tropical countries. The presence of the worms in the body causes diarrhea, liver damage, anemia, and a lowering of the body’s resistance to other diseases. Fertilized eggs pass out with the feces. Eggs released into the water hatch into free-swimming larvae. If a larva infects a snail, it undergoes additional asexual reproduction and produces a second larval stage. A single infected snail may be the source of thousands of larvae. These new larvae swim freely in the water. If they encounter a human, the larvae bore through the skin and enter the circulatory system, which carries them to the blood vessels of the intestine (figure 23.15).
FIGURE 23.15. The Life History of Schistosoma mansoni
(1) Cercaria larvae in water penetrate human skin and are carried through the circulatory system to the veins of the intestine. They develop into (2) adult worms, which live in the blood vessels of the intestine. (3) Copulating worms are shown. The female produces eggs, which enter the intestine and leave with the feces. (4) A miracidium larva within an eggshell is shown. (5) The miracidium larva hatches in water and burrows into a snail (6), where it develops into a mother sporocyst (7). The mother sporocyst produces many daughter sporocysts (8), each of which produces many cercaria larvae. These leave the snail’s body and enter the water, thus completing the life cycle.
In North America, people often encounter this kind of parasite as swimmer’s itch. Swimmer’s itch is caused by the larval stages of a species of Schistosoma burrowing into the skin. However, the normal hosts for the parasite are species of ducks. Humans are not ducks, so the larvae that have burrowed into the skin die and a rash often develops. In sensitive individuals, the reaction can be extreme and require medical attention.
Tapeworms are parasites that have a completely different structure and way of life from flukes. The life cycle involves two hosts. One of the hosts is an herbivore and the other is an omnivore or a carnivore. Both hosts are usually vertebrate animals. An herbivore eats tapeworm eggs that have been passed from an infected carnivore host through its feces. The eggs contaminate the vegetation and are eaten along with the vegetation the herbivore uses for food. An egg develops into a larval stage, which migrates to the herbivore’s muscle and forms a cyst. When the herbivore is eaten by a carnivore, the tapeworm cyst develops into the adult form in the carnivore’s intestine. The adult worm is highly specialized. It has no digestive system, but it has hooks and suckers for holding onto the intestine of its host. It produces a series of sections, called proglottids, by asexual reproduction. Each proglottid has a complete set of reproductive organs. It is important not to confuse proglottids with the segments seen in other kinds of organisms. Proglottids are not segments, but are asexually produced buds. Mating takes place between proglottids within the intestine, and the proglottids become filled with fertilized eggs. The end proglottids break off and pass out with the feces. As the proglottids dry out, they rupture and disperse the eggs into the environment (figure 23.16).
FIGURE 23.16. The Life Cycle of a Tapeworm
The adult beef tapeworm lives in the human small intestine. Proglottids, individual segments containing male and female sex organs, are the site of egg production. When the eggs are ripe, the proglottids drop off the tapeworm and pass out in the feces. If a cow eats an egg, the egg will develop into a cyst in the cow’s muscles. When humans eat the cyst in raw or poorly cooked meat, the cyst develops into an adult tapeworm.
23.7. CONCEPT REVIEW
19. List three structural characteristics typical of flatworms.
20. Describe the life cycle of a fluke.
21. Describe the life cycle of a tapeworm.