A. Structure and Function of Flatworms
1. Flatworms are the simplest animals with bilateral symmetry.
2. The tissues in bilaterally symmetrical animals develop from three germ layers:
Ectoderm, Mesoderm, and Endoderm.
3. In flatworms, the layers are pressed against one another to form a solid body.
4. Because flatworms do not have a hollow body cavity between the endoderm and mesoderm,
they are acoelomates.
5. The acoelomate body plan gives flatworms the thin, dorsoventrally flattened bodies for which
they are named.
6. This body shape ensures that no cell in a flatworm is far from the animal’s external
7. Thus, the cells can exchange oxygen and carbon dioxide directly with the environment
through diffusion, allowing flatworms to survive without a circulatory or respiratory system.
8. Like cnidarians, most flatworms have a gastrovascular cavity, which is a gut with a single
opening, and a mouth.
9. Food is taken in and digested in the gastrovascular cavity, and any digested material is
eliminated through the same opening.
10. Most of the sensory organs and nerve cells of flatworms, such as the marine species, are
located at the anterior end of the body. This characteristic is known as cephalization.
11. The classification of Platyhelminthes has undergone many recent changes.
12. Currently, the more than 20,000 species of flatworms are divided into four classes.
13. The four classes are: Turbellaria, Trematoda, Monogenea, and Cestoda.
14. Trematodes, monogeneans, and cestodes live as parasites on or inside other animals.
15. Almost all turbellarians are non parasitic, free-living organisms found in marine and
freshwater habitats and in moist terrestrial environments.
16. Parasitic flatworms probably evolved from free-living organisms.
17. As parasites evolved, some organs that were advantageous to free-living became modified for
parasitism, while other organs were lost entirely.
B. Class Turbellaria
1. The majority of the approximately 4,500 species in the class turbellaria live in the ocean.
2. However, the most familiar turbellarian is the freshwater planarian Dugesia.
3. Planarians have a spade-shaped anterior end and a tapered posterior end.
4. They move through the water by swimming with a wave-like motion of their body.
5. Over solid surfaces, planarians glide on a layer of mucus that they secrete, propelled by the
Cilia that cover their bodies.
Digestions and Excretion in Planarians
Planarians feed by scavenging for bits of decaying plant or animal matter. They also prey on smaller organisms, such as protozoa.
Food is ingested through a muscular tube called the pharynx, which the planarian extends from the middle of it’s body. The pharynx leads to the highly branched gastro vascular cavity. Cells lining the cavity secrete digestive enzymes and absorb nutrients and small pieces of food. The nutrients then diffuse to other body cells.
Organisms that live in fresh water must deal with the water that constantly enters their bodies by osmosis. Planarians eliminate excess water through a network of excretory tubules that run the length of the body. Each tubule is connected to several flame cells, which are so names because they enclose tufts of beating cilia that resemble flickering candle flames. The beating of cilia in the flame cells draws in the excess water. The water is then transported through the tubules and excreted from numerous pores scattered over the body surface. Neural Control in Planarians
The planarian nervous system is more complex than the nerve net of cnidarians. Two clusters of nerve cells at the anterior end, the cerebral ganglia, serve...
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