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.

• Wellcome to StackExchange Biology. I've tidied up your question a little. We always like links to sources people mentioned, so I've included what I imagine your source is to make it clear there was only a small section on your topic (which is best included in the body of the question as well as the title). Commented Mar 4, 2017 at 18:00
• This blog post provides examples of biradial and trimeric symmetry: planetfuraha.blogspot.com/2010/04/illustrating-symmetry.html Commented Jul 27, 2019 at 0:43

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.

E.g.ctenophores.

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.

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.

Imagine in terms of letters.

A has a bilateral symmetry. Left and right part are symmetrical.

X has a radial symmetry (Tetramerous), anterior part (v) is symmetrical to posterior part (^) but also identical to left part (>) and right (<) as well which are made by a different plane.

Now consider I. It has two bilateral symmetry but they are dissimilar. That is anterior part (T) is symmetrical to posterior part ( ⊥) but different from left part (]) which is symmetrical to right ([). This is biradial symmetry.

Basically, it is a radial symmetry with only 2 planes, contrast to a hydra, with infinite plane along central axis and starfish which is pentaradial, i.e, exacly 5 planes.

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.

• Your explanation illustrates nicely and simply the definition in OP's reference, but how does that apply to a typical shape of a Ctenophore, such as this example? It doesn't seem that II and IV would be identical, because II would contain a tentacle stem while IV would not. Commented Mar 5, 2017 at 9:09
• ts because that picture is from the side, from above it makes more sense, lanwebs.lander.edu/faculty/rsfox/invertebrates/images/…
– John
Commented Mar 5, 2017 at 14:07