The first three have bilateral, the last has radial symmetry.
It is a two-layered organism, with a form varying from cylindrical to oval, and usually a radial symmetry.
The prototype of all these groups was an organism something like a Medusa, with a radial symmetry.
They have not, as is usually supposed, secondarily acquired their radial symmetry.
This is characteristic of the body organization of all Echinoderms, and is known as radial symmetry.
At first they are bilaterally symmetrical, their radial symmetry being acquired later.
Lamarck thus accounts for the production of the radial symmetry of the medusæ and echinoderms, his Radiaires.
|radial symmetry |
Symmetrical arrangement of parts of an organism around a single main axis, so that the organism can be divided into similar halves by any plane that contains the main axis. The body plans of echinoderms, ctenophores, cnidarians, and many sponges and sea anemones show radial symmetry. Compare bilateral symmetry.