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Many triploblastic animals have a coelom (from the Greek koilos, hollow), a body cavity that forms from tissue derived from mesoderm.
A coelom’s fluid cushions the suspended organs, helping to prevent internal injury. Animals possessing coeloms are sometimes called coelomates.
Other triploblastic animals have a hemocoel, a body cavity that forms between the mesoderm and endoderm. Animals with only a hemocoel once
were called pseudocoelomates (from the Greek pseudo, false) and the cavity, a pseudocoelom.
Finally, some triploblastic animals lack a body cavity altogether. These compact animals tend to have thin, flat bodies. Such animals don’t require an internal transport system: With bodies that are only a few cells thick, the exchange of nutrients, gases, and wastes can occur across the entire body surface. These animals are sometimes called acoelomates.
#Coelomates
#Pseudocoelomates