Radial symmetry

From Palaeos

Radial symmetry in animals, as commonly defined, refers to a circular body plan that can dissected at any angle through the midpoint and give two identical halves, as opposed to bilateral symmetry that has a set midline.

In practice, almost none of the animals generally described as "radially symmetrical" are truly so (i. e., possess an infinite number of possible lines of symmetry), as organ systems will impose some degree of order to the bodyplan. For instance, jellyfish, one of the classic examples of radial symmetry, possess four gonads arranged in a cross, so reducing the effective number of lines of symmetry to two. In practice, therefore, "radial symmetry" is used to refer to animals that lack an anterior-posterior axis (but see Ctenophora), and can be divided into more than two identical segments. Examples of recorded orders of symmetry are:

Triradial (three): generally unknown from modern taxa, but represented in the fossil record by members of Trilobozoa.

Quadraradial (four): found in Scyphozoa, Rugosa, etc.

Pentaradial (five): the basic number for crown Echinodermata, though it is secondarily modified in a number of taxa.

Hexaradial (six): found in Hexacorallia.

Octoradial (eight): found in Octocorallia.

Radial symmetry is believed to be the ancestral state for Eumetazoa, replaced by bilateral symmetry at the base of the Bilateria. However, it has been secondarily redeveloped in a number of bilaterian phyla, such as Echinodermata and Bryozoa.