Evolution and Symmetry in the Order of the Sea-pens, i 1 L 



indeed, seem to have become rigid or invariable in the cases quoted. No 

 reason can, at present, be given to account for this rigidity of certain 

 characters in a few of the many thousands of sedentary individuals and 

 colonies, but they cannot be put forward as a controversion of the general 

 statement that sedentary animals are usually more variable as regards their 

 principal organs than animals that are free and active in their movements. 



It is not surprising that bilaterally symmetrical animals should be less 

 variable than radially symmetrical animals. In order to secure the greatest 

 possible efficiency of movement in any direction it is necessary that the 

 organs should be equally poised on either side of the median plane. Any 

 variation of an organ on one side of the plane, unless balanced by an exactly 

 equivalent variation of the corresponding organ on the other side, vrould 

 produce deviation of direction in movement. An increase of the median 

 unpaired structures of the dorsal side would require some compensating change 

 in the structui'e of the unpaired or paired structures of the ventral side, 

 variations in size or form of the anterior end would require some modification 

 of the structure at the posterior end, if the mechanical economy is to be 

 maintained. There are many kinds of variation, therefore, which, in bila- 

 terally symmetrical animals, would materially interfere with the chances of 

 survival of the variant in the struggle for existence, but would not, at any 

 rate to the same extent, interfere with the chances of a radially symmetrical 

 sedentary or drifting animal. 



Moreover, it can be understood that many kinds of sedentary and drifting 

 animals have need of powers of variation and accommodation of form which 

 are not required by animals that are active. 



In most cases, sedentary animals begin life as free swimming or floating 

 larvae. These larvae settle down in positions of various environmental con- 

 ditions, and there they remain for life or perish. 



Considerable powers of variations or plasticity in growth are therefore of 

 immense advantage to a species for accommodation to the various and the 

 variable conditions of the environment. 



A sedentary animal that does exhibit considerable variability and accom- 

 modation has obviously a better chance of survival than one that is more 

 rigid in its development and growth. 



With an animal that is capable of vigorous muscular movements through 

 its medium such variability has not the same essential value, for within 

 limits it can escape from unfavourable environments and seek others that are 

 better suited to its more rigid characters. Sedentary animals are either 

 solitary or colonial in habit. Those that are solitary are almost invariably 

 radially symmetrical, but those that are colonial, although showing radial 



