[Hydromedusae] The main radial axes of a medusa, corresponding in most species to the radial canals.

[Cubo-/Scyphomedusae] The perradial radii are the four main radii of the medusa, as though North, South, East and West. In between (SE, NE, etc.) occur the four interradii; and in the radii between the others (ESE, ENE, etc.), the 8 adradii. Cubomedusae are all tetraradiate and the types of radii are clear in all species, but in some of the three other groups some or many genera and families have more than four radii and identification of their homologies must be traced carefully. The following remarks thus apply essentially to adult forms which are tetraradiate, which in all groups is regarded as fundamental. The stomach pouches are taken to be homologous between the groups, and perradial in position. Thus in Cubomedusae the stomach pouches and rhopalia are on the perradii, and the pedalia are interradial; in coronates the stomach pouches and the corners of the mouth are perradial, and the rhopalia may be perradial or interradial, depending on the family; in semaeostomes the gastric pouches and the corners of the mouth (prolonged into the mouth-arms) are perradial, and the gonads interradial; and in rhizostomes the stomach pouches are perradial, the gonads interradial, and the 8 mouth-arms are regarded adradial.

Alternative forms for Perradial : adradial, In Cubo-/Scyphomedusae:, interradial.