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Bilateral Symmetry and Cephalizatio1

Bilateral Symmetry and Cephalizatio1 - rise to muscles and...

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Bilateral Symmetry and Cephalization: Phylum Platyhelminthes The phylum Platyhelminthes contains about 13,000 species of flatworms subdivided into three classes: two parasitic and one free-living. The planaria and relatives are freshwater animals placed in the class Turbellaria. Flukes are external or internal parasites belonging to the the Class Trematoda. Tapeworms are internal parasites and form the Class Cestoda.The phylum as a whole has adult bilateral symmetry and cephalization (the development of a head with sensory organs, in most members). Flatworms have three tissue layers: ectoderm, mesoderm and endoderm and a body plan that is acoelomate and sac-like with a single opening. The mesoderm layer gives
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Unformatted text preview: rise to muscles and reproductive organs. Free living forms have muscles, a nerve cord, and digestive organs, but lack both the respiratory and circulatory systems common to the so-called "higher" animals (in other words like ourselves). Flatworms, as shown in Figure 18, have a branched gastrovascular cavity that is the site of extracellular digestion and which distributes nutrients throughout the body. Gas exchange occurs by diffusion through the skin. Platyhelminths have an excretory system that also functions as an osmotic-regulating system. Flatworms have a ladder-style nervous system composed of paired ganglia that form a brain connected via nerve cells to sensory cells in the body wall....
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