108 



Croonian Lecture : Evolution and Symmetry in the Order of the 



Sea-j^ens. 

 By Prof. Sydney J. Hickson, F.RS. 



(Lecture delivered June 22, 1916, — MS. received November 19, 1917.) 



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 bilater- 

 ally 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 completelj' 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. 

 Eadially symmetrical animals may be spherical or oval, dome- or disc-shaped, 

 or cylindrical in form, and they do not exhibit anterior and posterior extremi- 

 ties, dorsal and ventral surfaces, nor right and left sides. 



Animals that are bilaterally symmetrical are usually free, and propel their 

 bodies actively through the medium in which they live by powerful muscular 

 movements of the body wall or of the appendages. Animals that are radially 



