This is a mature moon jelly with a diameter nearly 30cm. There are at less 12 type of moon jelles on the earth, with slightly different characters including the folding of oral arms, oral arm width, distal gastric diameter, bell shape, etc.
Rhopalium, indicated by yellow arrow.
Rhopalium are the main sensory structures of scyphozoan (true-jellyfish). Light-sensory structure (pit eye), and gravity-sensory structure (statoliths) were included inside a rhopalia.