I. INTRODUCTION. 



The conclusions which are set forth in the following pages concerning the biological 

 significance of unsymmetrical form and structure in a number of organisms have 

 been reached in carrying on two very distinct lines of work. The first was a series 

 of researches on the behavior and reactions of the Infusoria (Jennings, '97-: 02), in 

 which the relation of the prevailingly unsymmetrical form of these creatures to their 

 method of life and their reactions was strongly impressed upon me. The second was 

 a systematic study of a family of Rotif era, the Rattulidse, the results of which have 

 not hitherto been published.* The Rattulidse have an unsymmetrical form, and 

 in searching for the significance of this asymmetry I was again led to recognize its 

 relation to the behavior and method of life of these creatures. I shall draw upon 

 both these lines of work in the present paper, giving in especial detail the results on 

 the Rattulidae. 



Before inquiring as to the biological significance of asymmetry, it will be well to 

 recall the generally accepted views as to the significance of radial and bilateral sym- 

 metry among organisms. 



Radial symmetry is as a rule characteristic of fixed animals, such as Hydra^ 

 Metridium, corals, crinoids, and the like. In such organisms the body extends freely 

 from the substratum, and all sides of the animal come into similar relations with the 

 environment. There is thus no reason for the sides becoming different; dorsal and 

 ventral surfaces are not developed, and the animal remains radially symmetrical. 

 Where freely moving animals have retained radial symmetry, it is usual to explain 

 this as due to their recent origin from fixed forms. 



The differentiations shown by bilaterally symmetrical animals are similarly 

 brought into relation theoretically with their methods of life and movement. Ante- 

 rior and posterior ends differ because they come into different relations with the 

 environment, owing to the forward movement. In the same way dorsal and ventral 

 surfaces differ because they come into different relations with the environment, — the 

 ventral surface being more commonly in contact with a surface, the dorsal not thus 

 in contact, but subjected to the light and other influences coming from above. On 



* The detailed systematic results of this work are to appear in the Bulletin of the U. S. Fish Commission. 



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