OOTINEON LINDAHLI. 471 



three octomeral larvse of phylogenetic value with diflereut 

 muscular arrangements, which at first sight is unlikely. 

 That the eight mesenteries of one type are all homologous 

 with those of another is contradicted by a glance at the 

 diagrams given above ; they have different muscle-relations, 

 and hold different positions in the twelve-rayed and adult 

 stages. An objection of less weight is that if there exist 

 that detailed homology which would follow from the value 

 assigned to them by McMurrich and Boveri, mesenteries 

 holding similar positions should appear in the same order 

 in both cases, but obviously do not. To refer the differences 

 between these two types to a " Vereinfachung," or to an 

 " abbreviation " of development, is no explanation, and may 

 be made to cut equally well in two directions. 



Again, there arise the cases of the Monaulese (Hertwig, 

 'Report on the Actiniaria,' Supplement; Chall. Rep. Zool., 

 xxvi) and of the Holactiniae (Boveri, '' Das Genus Gyractis, 

 eine radial-symmetrische Actiuien-form," ' Zool. Jahrbiicher,' 

 Abth. Syst., vii, 241). 



Fig. C. Fig. D. 



Diagrams of tlie mesenterial relations of the Monaulese (C) and of the 

 Holactinias (D). 



The essential difference between these and the Hexactinise 

 lies in the fact that one pair only of ' directive ' mesenteries 

 is present in the first group, none (according to Boveri's view) 

 in the second. Where in these can we look for homology 

 with the Hexactinise? Is the single pair of directives of the 



