acoelomates

n. organisms that have no coelom or no body cavity (tapeworm, flatworm, fluke, ribbon worm, jellyfish)

n. animal or organism with no coelom or no body cavity

By the broadest definition, a body cavity is any fluid filled space in a multicellular organism. However, the term usually refers to the space, located between an animal’s outer covering (epidermis) and the outer lining of the gut cavity, where internal organs develop. "The body cavity" of human body cavities normally refers to the ventral body cavity, because it is by far the largest one in area.
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