10 HlCKSON, Animal Symmetry. 



two examples of sedentary colonies that look alike, when 

 placed side by side, may belong to totally different groups 

 of the animal kingdom. 



Many examples could be chosen to illustrate this 

 point, but I will mention one which claimed my attention 

 a few months ago. There are two species described by 

 Jullien and Calvert named Hornera eburnea and H. verru- 

 cosa, and placed by them in the Order of the Polyzoa 

 In examining the Polyzoa collected by the "Siboga" 

 Expedition, Dr. Harmer came across specimens of the 

 same species, but feeling some hesitation about their 

 systematic position sent them to me for examination. A 

 study of the minute structure of the specimens leaves no 

 doubt whatever in my mind that they are as Dr. Harmer 

 suggested, not Polyzoa at all, but members of the order of 

 Stylasterina. 



The factors that determine the ultimate form and 

 symmetry of a sedentary colony are probably two, at 

 least, in number. There is a factor determining the 

 general plan on which the colony is to be built up, and 

 there is a factor of plasticity which enables this plan to 

 be modified in various ways according to the environment. 

 Probably these two factors occur in the determination of 

 the exact form of every animal, but the essential point 

 about the sedentary colony is that the factor of plasticity 

 is one of extreme importance as compared with its im- 

 portance in bilaterally symmetrical animals. The reason 

 for this is obvious. A bilaterally symmetrical animal by- 

 its power of locomotion from one locality to another can, 

 to a certain extent, choose its own environment, but a 

 sedentary colony, when once it has fixed itself to its 

 support, must accommodate itself to the ever-changing 

 conditions of environment. A large measure of plasticity, 

 in other words, is a condition of its survival. If it were 



