NSCI 1403 Lecture Notes - Lecture 18: Rumen, Radula, Cud
27 Dec 2016
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Document Summary
Most animals need to digest food into small molecules that can cross plasma membranes. Digestion provides the energy needed to carry out routine metabolic activities and maintain homeostasis. The digestive tract ingests food, breaks down food into small molecules that can cross plasma membranes, absorbs these nutrient molecules, and eliminates nondigestible remains. Planarians are organisms with an incomplete digestive tract, a single opening (usually called a mouth). Planaria are carnivorous and feed largely on smaller aquatic animals. The digestive system contains only a mouth, a pharynx, and an intestine. To feed, its pharynx extends far beyond the mouth to suck up minute quantities at one time. Digestive enzymes in the tract allow some extracellular digestion. Digestion is finished intracellularly by cells that line the digestive cavity; food then diffuses to nearby cells. The digestive system lacks regions of specialized function.
