156 McMURRICH. [Vol. V. 



division into forms having a skeleton and forms without a skel- 

 eton, may not be possible, as even the closer limitation of the 

 Hexactinias given here does not exclude the possibility of many 

 of their families having more affinity to single families of corals 

 than to other Hexactiniag." The observations which have been 

 recorded since the publication of Hertwig's paper only tend to 

 confirm his opinion. The arrangement of the mesenteries, and 

 their order of appearance, demonstrate conclusively that the 

 majority, if not all, of the Hexacorallia are closely related to the 

 Hexactiniae. 



As regards the Tetracorallia or Rugosa, however, matters 

 are in a very unsatisfactory state. We know nothing as to their 

 soft parts, and opinions differ as to the mode of arrangement of 

 the septa. Some authors have been inclined to regard the 

 Rugosa as the ancestral forms of the Hexacorallia, and have 

 found among recent corals what they believed to be connecting 

 links between the two groups. All such theories have, however, 

 proved unsatisfactory, and for a discussion of their value Neu- 

 mayr's recent work ('89) may be referred to. The trouble is 

 that too much importance has been attached to adult resem- 

 blances, and too little to the essential point, the manner of 

 formation of the mesenteries and septa. 



There seems to be little room for doubt that the mesenteries 

 of the Rugosa increased in a bilateral manner. Whether the 

 primary plan of the organism was hexamerous or tetramerous is 

 the doubtful point. The latter view was maintained by Kunth 

 ('69), and is in harmony with the marked tetrameral symmetry 

 which so many Rugosa present. According to Kunth, the 

 formation of secondary septa was limited to four points, one on 

 either side of the dorsal (fossa!) septum, and one on each side 

 immediately ventral to the lateral septa. 



R. Ludwig, and later, Pourtales ('71), maintained the other 

 view. Pourtales reached his conclusion from a study of LopJw- 

 phyllum, and while he found that the formation of secondary 

 septa was limited to four regions, he believed that six primary 

 septa were present, and that in adult forms the arrangement 

 was a modification of an hexamerous condition. 



Whichever of these two views be accepted, it seems certain 

 that the mode of formation of septa, and therefore of mesen- 

 teries, in the Rugosa, is entirely different from that occurring 



