A mesotubarium is a specialized anatomical structure found in certain invertebrates, particularly within the phylum Cnidaria, such as jellyfish and some polyps. It serves as a supportive and structural element, helping to maintain the shape of the organism's body and facilitating the movement of nutrients and other substances. The mesotubarium is typically composed of a jelly-like substance and plays a role in the overall physiology and functionality of these marine organisms.