axial organ

The axial organ is a complex tissue mass found in all echinoderms except for holothurians. It serves as a central connection point for the perivisceral coelom, the water-vascular system, and the hemal system. While its exact functions are still being researched, the axial organ is known to play a role in defending against foreign organisms, possesses contractile abilities, facilitates fluid circulation, and may be involved in excretion and secretion. In echinoderms, many major organ systems, including the water-vascular system, muscles, and hemal system, exhibit a pentamerous (five-part) arrangement, which is also reflected in their skeletal structure.12