No. 3.] THE MORPHOLOGY OF THE ACTINOZOA. 121 



Thaumactis medusoides (Fowler, '88) in the new order and 

 leave the other out; and so with Sagartia undata (Carlgren, 

 '93). This, it seems to me, is a reductio ad absurdnm. 



The complete absence of directives is not necessarily a phy- 

 logenetic peculiarity, but one extreme of the same tendency to 

 an irregularity in the number of the directives of which many 

 examples are now known. There seems to be no more reason 

 for considering the complete absence of directives a peculiarity 

 of ordinal importance than there is for so regarding their 

 reduction to a single pair or their increase to three or four 

 pairs. It is rather a peculiarity which is essentially individual, 

 possibly rising secondarily in some cases to the value of a 

 specific or even a generic character, and to regard it as of 

 ordinal value is, it seems to me, inconsistent with our present 

 ideas as to the phylogeny of the Anthozoa. 



In conclusion it may be pointed out that the irregularities 

 which have been described in the arrangement of the mesen- 

 teries of Gyractis and Thalassianthus are just what might be 

 "expected to occur in connection with the absence of directives, 

 since we usually find considerable departure from the normal 

 arrangement even in forms which lack only one pair (cf. 

 Thorell, '58, Carlgren, '93, and Parker, '97). 



UNIVERSITY OF MICHIGAN, 

 June 10, 1897. 



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