132 THE SHAPES AND SIZES OF ANIMALS 



outline instead of circular ; there will now be only 

 two planes of symmetry, which will divide the 

 animal into exactly similar and corresponding 

 halves; one of these planes passing along the 

 longer diameter of the elliptical mouth, the other 

 along its shorter diameter. Next imagine the 

 mouth, instead of being elliptical, to be ovoid or 

 egg-shaped in outline, with a larger and a smaller 

 end. There is now only one single plane of 

 symmetry possible, namely, that passing along the 

 longer diameter of the mouth opening, for any 

 other plane will divide the mouth, and therefore the 

 animal, into two unlike halves. 



The origin of this biradiate symmetry is a little 

 obscure. There are reasons for thinking that it 

 first arose in colonial forms, such as Alcyonium 

 or Pennatula, inasmuch as in these colonial forms 

 it is very well marked, and furthermore the plane 

 of symmetry of the individual polypes always has 

 a definite relation to the axis of the entire colony, 

 and the differences between the two sides of the 

 animal on which the biradiate symmetry depends 

 seems to be associated with a special provision for 

 securing a rapid and efficient circulation of water, 

 not merely through the individual polypes them- 

 selves, but throughout the whole colony. This 

 explanation seems fairly satisfactory in most cases. 

 It must be noted however that it involves the 

 descent of solitary forms, such as Cerianthus and 

 many other Anemones in which biradiate symmetry 

 is well marked, from colonial ancestors, a line of 

 ancestry for which there is but little independent 



