Structure And Function Of Flatworms

Flatworms are the simplest animals with bilateral symmetry. The tissues in bilaterally symmetrical animals develop from three germ layers: ectoderm, mesoderm, and endoderm. In flatworms, the three germ layers are pressed against one another to form a solid body. Because flatworms do not have a hollow body cavity between the endoderm and the mesoderm, they are acoelomates.

The acoelomate body plan gives flatworms the thin, dorsoven-trally flattened bodies for which they are named. This body shape ensures that no cell in a flatworm is very far from the animal's external environment. Thus, the cells can exchange oxygen and carbon dioxide directly with the environment through diffusion, allowing flatworms to survive without a circulatory system or respiratory system. Like cnidarians, most flatworms have a gastrovascular cavity, which is a gut with a single opening, and a mouth. Food is taken in and digested in the gastrovascular cavity, and any undigested material is eliminated through the same opening. Most of the sensory organs and nerve cells of flat-worms, such as the marine species shown in Figure 34-1, are located at the anterior end of the body. This characteristic is known as cephalization.

The classification of Platyhelminthes has undergone many recent changes. Currently, the more than 20,000 species of flat-worms are divided into four classes: Turbellaria, Trematoda, Monogenea, and Cestoda. Trematodes, monogeneans, and ces-todes (SES-tohdz) live as parasites on or inside other animals. Almost all turbellarians are nonparasitic, free-living organisms found in marine and freshwater habitats and in moist terrestrial environments. Parasitic flatworms probably evolved from free-living organisms. As parasites evolved, some organs that were advantageous to free-living became modified for parasitism, while other organs were lost entirely.

objectives

• Summarize the distinguishing characteristics of flatworms.

• Describe the anatomy of a planarian.

• Compare free-living and parasitic flatworms.

• Diagram the life cycle of a fluke.

• Describe the life cycle of a tapeworm.

vocabulary pharynx flame cell cerebral ganglion eyespot fluke tegument primary host intermediate host schistosomiasis scolex proglottid cyst

figure 34-1

Many of the sensory organs in this marine flatworm, of the genus Eurylepta, are concentrated in the two tentacles at the anterior end of its body. This characteristic is an example of cephalization.

figure 34-1

Many of the sensory organs in this marine flatworm, of the genus Eurylepta, are concentrated in the two tentacles at the anterior end of its body. This characteristic is an example of cephalization.

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