Coelom is absent in the class of animals termed

Coelom is absent in the class of animals termed coelenterata, also known as cnidarians. Cnidarians are a diverse group of aquatic animals that include jellyfish, corals, and sea anemones. They have a simple body plan with two cell layers, an outer epidermis and an inner gastrodermis, separated by a jelly-like layer called mesoglea. They do not have a true body cavity or coelom. Instead, their digestive and reproductive organs are suspended in the gastrovascular cavity, which functions both for digestion and distribution of nutrients throughout the body. This cavity is lined by gastrodermis, which performs both digestive and absorptive functions. In contrast, the other animal groups listed, Mollusca, Reptilia, and Arthropoda, all have a true coelom, which is a fluid-filled body cavity completely lined by mesoderm, the middle layer of cells in the embryo. This coelom allows for more complex body structures and organ systems to develop, and also serves as a space for circulation of fluids and other substances in the body.