![]() The excretory system has protonephridia with the flame. In fact, the respiration generally occurs by simple diffusion through the body surface. Respiratory and circulatory systems are absent.In a few species, the digestive system is completely absent. In fact, there is a single opening which leads to a well-developed gastro-vascular cavity. The digestive system is incomplete or absent.They exhibit an organ system grade of organization. ![]()
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