NO. 7 SEA-LILIES AND FEATHER-STARS CLARK 33 



Flowers facing directly upward or directly downward are always 

 regular, insuring maximum visibility from all directions ; irregular 

 flowers always are directed more or less laterally. Crinoids which 

 rest on muddy bottoms and therefore face directly upward (as Penta- 

 metrocrimis) or which have pendent crowns (as Ptilocrinus) are also 

 symmetrical since this means maximum efficiency in combing the 

 water which may pass across them in any direction. 



Among our native flowers (in eastern North America) the various 

 colors represented fall into three distinct groups on the basis of the 

 proportion of irregular to regular flowers included, as follows : 



Group I : About One-Quarter of the Flowers Irregular 



Red White Green 



Per cent of regular flowers 78 y2) 72 



Per cent of irregular flowers 22 27 28 



Group II : Nearly One-Half of the Flowers Irregular 



Brown Yellow Orange 



Per cent of regular flowers 57 56 56 



Per cent of irregular flowers 43 44 44 



Group III : More than Three-Quarters of the Flowers Irregular 



Blue Purple 



Per cent of regular flowers 24 21 



Per cent of irregular flowers 76 79 



The proportion of variegated flowers is much higher in irregular 

 than in regular types since in the irregular types every artifice which 

 will increase the visibility must be adopted. The proportion of varie- 

 gated crinoids is very much higher in the irregular than in the regular 

 species because they are all from shallow water and it is in the shallow 

 water that the colors of crinoids are best developed. From a mini- 

 mum in the coldest regions the proportion of irregular flowers in- 

 creases to a maximum in the tropics ; from a minimum in the coldest 

 water the irregular crinoids increase to a maximum in the tropical 

 littoral. 



About three-quarters of all our blue, violet and purple flowers are 

 irregular; since blue, violet and purple seem to be the most con- 

 spicuous colors so far as insects are concerned a flower loses less in 

 visibility by being of these colors than it would by being of other 

 colors. Blue is only recorded from irregular crinoids, and violet 

 and purple are much more common in irregular than in regular types, 

 as a result of the occurrence of the former only in shallow water. 



In the crinoids the pigment is not confined to the exterior of the 

 animal as in most active types, including the other echinoderms, but 



