MORPHOLOGY OF ANIMALS 



IV. ORGANIC SYMMETRY 



Many inorganic things, such as molecules and crystals, 

 have varying forms of symmetry depending upon the spatial 

 relation of their atoms and molecules. Likewise the con- 

 stituent parts of an organism, such as cells, organs, antimeres, 

 and metameres, bear certain spatial relations to one another, 

 and this is what is meant by organic symmetry or asymmetry. 



In any solid body there are typically three axes in the 

 three dimensions of space and each of these axes has two 

 poles. If all the axes and poles are alike we have spherical 

 symmetry, a condition which is found in few if any living 

 things. When the two poles of one axis differ from each 

 other (polar differentiation) while all the other poles and 

 axes are alike we have radial symmetry, a condition which is 

 found in many egg cells as well as in some adult organisms. 

 When all three axes differ from one another, while the two 

 poles of only two of these axes differ from each other, we 

 have bilateral symmetry, a condition which is found in a 

 great number of plants and animals. These and other forms 

 of symmetry are summarized in the following table : 



SYMMETRY 



ANIMAL 

 EXAMPLES 



AXES POLES 



Homaxial Homopolar 



Single heteraxial Single heteropolar 

 Triple heteraxial " " 



Double 

 " " Triple " 



Probably the higher forms of symmetry and asymmetry 

 were originally derived from the lower forms by progressive 



n 11 : 



1. Spherical 



(ideal) 



2. Radial 



3. Biradial 



4. Bilateral 



5. Asymmetry 



Few if any 



Hydra, jellyfish 



Ctenophores 



Almost all animals 



above Ctenophores 



Paramecium, Snails 



