All cnidarians have two tissue layers. The outer layer is called the epidermis, whereas the inner layer is called the gastrodermisand lines the digestive cavity. Between these two layers is a non-living, jelly-like mesoglea. There are differentiated cell types in each tissue layer, such as nerve cells, enzyme- secreting cells, and nutrient-absorbing cells, as well as intercellular connections between the cells. However, organs and organ systems are not present in this phylum.
The nervous system is primitive, with nerve cells scattered across the body in a network. The function of the nerve cells is to carry signals from sensory cells and to contractile cells. Groups of cells in the nerve net form nerve cords that may be essential for more rapid transmission. Cnidarians perform extracellular digestion, with digestion completed by intracellular digestive processes. Food is taken into the gastrovascular cavity, enzymes are secreted into the cavity, and the cells lining the cavity absorb the nutrient products of the extracellular digestive process. The gastrovascular cavity has only one opening that serves as both a mouth and an anus (an incomplete digestive system). Like the sponges, Cnidarian cells exchange oxygen, carbon dioxide, and nitrogenous wastes by diffusion between cells in the epidermis and gastrodermis with water.
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