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PRECAMBRIAN PERIOD

660-630 million years ago

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cnidarian with digestive system

Cnidarians do not possess organs or systems, but they possess a number of significant characteristics of the digestive systems of higher animals. They possess a mouth and a gastrovascular cavity. The food which will be digested can be trapped by mucus and moved to the gastrovascular cavity by the action of cilia. Digestive enzymes are secreted into cavity, mostly proteinases from endodermal gland cells. Muscle cells surround the gastrovascular cavity and may be present in multiple layers. Some cnidarians have a pharynx. Some ctenophores possess anal canals which means that indigestible material does not need to exit the body through the mouth, which is the condition of most cnidarians. In cnidarians, extracellular enzymes don't break down the food completely-small pieces undergo phagocytosis and are digested further intercellularly (Hickman 185, Fretter). Hydra possess 2 kinds of gland cells in their digestive tracts for the production of mucus and enzymes. The use of extracellular digestion by primitive animals probably introduced a selective advantage for greater differentiation of the gastrointestinal tract. Although extracellular digestion can aid intracellular digestion, eventually it requires a control of the pH in the area of digestion, the ability to mix food and enzymes, and the separation of the digestive area into separate compartments to function well (Beklemishev). Cnidarians can digest most types of biomolecules but most cannot digest starches (Hickman, 137; Hyman). Cnidarian digestive enzymes include a trypsin-like digestive enzyme that functions in an alkaline environment (Hyman, 393).