Croonian Lecture: Evolution and symmetry in the order of the sea-pens

Abstract

1. On Radial and Bilateral Symmetry in the Animal Kingdom . In a general survey of the animal kingdom two kinds of body symmetry are found—the bilateral and the radial. In many cases, genera, families, and even classes of animals show some structural departures from the completely radial or bilateral symmetry; in others, there is a combination of the two symmetries, as when an outer body form of radial symmetry covers bilater­ally symmetrical organs, and, further, there is now conclusive evidence that in the course of the evolution of certain groups of animals one form of body symmetry has been supplanted by the other. With all the varieties of form and structure adapted to the different conditions of life, and with all the complexities of development and organisation due to phylogeny, there are but few examples of animals that are completely bilateral or completely radial as regards all their organs, but the dominance of one or other of these symmetries is, in most cases, so far pronounced that any animal can be placed in its proper group on this method of classification. It is not possible to give in a few words a comprehensive definition of what is meant by the two symmetries; but if attention is confined for the moment to such types as the earthworm or a fish on the one hand, and to a polyp or a jelly-fish on the other hand, a basis for a definition may be found for each of the two symmetries. Thus, a bilaterally symmetrical animal is one in which the principal organs and appendages of the body are arranged in pairs on either side of a median vertical plane. Such an animal exhibits an anterior and a posterior extremity, a dorsal and a ventral surface, and a right and left side. And a radially symmetrical animal is one in which the principal organs and appendages of the body are arranged symmetrically on radial lines or planes proceeding from a common centre or a common axis. Radially symmetrical animals may be spherical or oval, dome- or disc-shaped, or cylindrical in form, and they do not exhibit anterior and posterior extremi­ties, dorsal and ventral surfaces, nor right and left sides.