The Actiuiae of the Plate C'ollection. 217 



Word as emploj^ed by Milne-Edwards and in tliis sense it was also 

 employed by R. Heetwig in 1882 and 1888 and by varions later 

 authors such as Haddon, Carlgren, Duerden and myself. In 1898, 

 lioweA^er, it was employed by Van Beneden with a very extensive 

 modiflcation of its meaning, botli tlie Zoantheae and Ceriantheae 

 being- witlidrawn from it; in the following year it was used by 

 Carlgren (1899) in the same sense and it is in this sense that I 

 iise it here. 



It includes, consequently, the Orders Edwardsiae Protactiniae 

 and Hexactiniae of my earlier Classification (1893), a fiision of these 

 Orders being demanded by the recent work demonstratiug- the exis- 

 tence in certain Edwardsiae of a Hexactinian type of strncture. In 

 one respect the group seems likely to require modiflcation in the 

 future to the extent of includiug in it certain of the Madreporaria, 

 as was suggested by R. Hertwig. Onr knowledge of the corals in- 

 dicates strongly a necessity for regarding the presence or absence 

 of a coralium as a quite subordinate matter, but since it seems prob- 

 able from Dueeden's recent work on Pontes that the Madreporaria 

 do not constitute a homogeneons group and until we are in possession 

 of definite Information as to whether the relations of the hexamer- 

 ous corals to the Actiniaria are monophyletic or polyphyletic, it 

 seems advisable to retain the two groups separate. 



A concise definition of the group as thus limited is rather diffi- 

 cult, owing to the variations presented by different forms from al- 

 niost every one of the features which may be regarded as typical. 

 It may he stated however, in some such words as these. 



Anthozoa, solitary or but rarely approximating to a colonial con- 

 dition ; destitute of a coralium. The meseuteries not less than eight 

 in number constituting four primary couples arranged symmetrically 

 on either side of the sagittal axis ; four of these meseuteries situated 

 in pairs at each extremity of the sagittal axis, have, as a rule, their 

 longitudinal muscles on the exocoelic faces and form what are termed 

 the directives meseuteries. Additional meseuteries belonging to 

 one or more sets which appear successively, may be present; the 

 first set consists of two secondary couples which form pairs having 

 their longitudinal muscles on the endocoelic faces with the lateral 

 meseuteries of the primary couples and thus complete a primary cycle 

 of six pairs, the second set cousists of a couple or two, four or six 

 pairs with their longitudinal muscles on their endocoelic faces 

 occuring in two or more of the primary exocoels (rarely the 



15* 



