No. i.] MORPHOLOGY OF THE ACTINOZOA. 157 



in the Hexacorallia, and accordingly it is exceedingly unlikely 

 that there can be any intimate relation between the two groups. 



In the determination of the relations of the Rugosa to the 

 other Actinozoa, however, the question whether their funda- 

 mental plan is tetramerous or hexamerous becomes of great 

 importance. In the former case it may be supposed that eight 

 primary mesenteries (sarcosepta) were present, the Rugosa 

 agreeing in this respect with the Alcyonaria and the Edwardsiae. 

 They differ from these, however, in that secondary mesenteries 

 are added in the course of development, as in the Ceriantheae ; 

 but instead of developing at one region, as in the forms of that 

 group, they arise singly and bilaterally at four distinct regions, 

 one mesentery corresponding to each septum of the secondary 

 series. 



If, however, the primary septa were six in number, it must be 

 supposed that there were twelve primary mesenteries, the septa 

 being entoccelic. Whether the secondary mesenteries devel- 

 oped singly or in pairs, approaching the arrangement found in 

 Peachia, though not like that form derived from an Hexactinian 

 condition, it is impossible at present to say. 



Kunth's view of the development of the septa is, however, 

 the one which seems to prevail among palaeontologists at pres- 

 ent, and is that accepted by Zittel in his Handbook, and in the 

 recent publication by Neumayr. It is to be hoped that the 

 question will be definitely decided in the near future by renewed 

 and extensive observations. In the meantime, it will be most 

 in harmony with our information on the subject if we consider 

 the Rugosa one of the most primitive groups of Actinozoa, more 

 primitive, perhaps, than even the Alcyonaria or Edwardsiae. 



The relative position of the Alcyonaria I have already referred 

 to. It seems not improbable that they are phylogenetically 

 antecedent to the Edwardsiae. The arrangement of the mesen- 

 terial musculature is, it seems to me, simpler ; and the slight 

 development of the siphonoglyphe appears to be a point of con- 

 siderable importance. The pinnate nature of the tentacles, the 

 peculiar arrangement of the mesenterial filaments, the axis and 

 spicules, and the simultaneous appearance in the ontogeny of 

 all the eight mesenteries, seem to present material difficulties 

 to the view I have advanced. It must be remembered, however, 

 that the Alcyonaria are a very highly specialized group. There 



