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Flatworms I. What Is a Flatworm? A) soft B) flattened C) has tissues and internal organ systems D) simplest animal to have three embryonic germ layers (endoderm, mesoderm, ectoderm) E) bilateral symmetry F) cephalization G) acoelomates, which means—no coelom. 1) coelom is fluid-filled body cavity, lined with tissue derived from mesoderm 2) digestive cavity is the only body cavity in a flatworm II. Form and Function in Flatworms A) thin and most of cells are close to external environment 1) rely on diffusion for respiration, excretion, and circulation B) Free-living flatworms have organ systems for digestion, excretion, response, and reproduction. BUT C) Parasitic species are simpler in structure—usually no organs for digestion, excretion & response III. Feeding A) single opening through which both food and wastes pass (both in and out—no digestive tract) 1) Near mouth is muscular tube called a pharynx 2) extend pharynx out of the mouth then pumps food into the digestive cavity BUT B) parasitic worms do not need a complex digestive system C) obtain nutrients from digested foods from host