The jellyfish endocrine system is almost mysterious. The jellyfish is composed of about 95% water. It also lacks protein and most common minerals found in animals. However, the jellyfish has sensory organs called rhopalia. Rhopalia forms small structures near the rim of the jellyfish. Located within the rhopalia are statocysts. This helps the jellyfish maintain hormonal balance. This is what makes the muscles of the jellyfish stay right side up.