I am trying to visualize biradial symmetry. I have read the section in the Wiki entry on Symmetry in Biology but it is quite brief and there is no example to illustrate it. I haven’t been able to find one anywhere else on the internet.
In biradial symmetry, in addition to antero-posterior axis there are also two other axes or planes of symmetry at right angles to it and each other such as the sagittal or median verticular-longitudinal and transverse or cross axes. Such animals have two pairs of symmetrical slides i.e there are two planes of symmetry.
You can visualize it as a combination of radial and bilateral symmetry. The body has similarity on either side of a central axis but slight differences in sections next to each other if divided across any plane.
In this image you can see that there are two planes of symmetry, one passing along the oral-aboral axis and the long axis of the mouth. The other passing along the oral-aboral axis and short axis of the mouth. The antimeres on either side of one plane are slightly different from the antimeres on either side of the plane (Imagine in 3D). The comb plates give it radial symmetry and mesogleal layer (jelly-like) give it bilateral symmetry.
If you are interested in the evolutionary aspect, then read this.
lets use a simple quadrant diagram to explain
so in this diagram 2 and 4 would be identical, but 2 and 1 or 3 would not.
or you could say side 2, 3 is the same as side 1,4 but not as a mirror image more as a rotation.
Another way you can picture it is radial symmetry but with only two sides, hence BIradial. Many believe it may be an intermediate stage between radial and bilateral symmetry.
Biradial symmetry is a type of symmetry in which there are two planes of symmetry passing through the principal axis. It is different from bilateral symmetry where there is only one plane of symmetry.
Another subtype of radial symmetry is pentamerous radial symmetry which has only 5 planes of symmetry, seen in star fish.