OOTINEON LINDAHLI. 469 



tinian, with twelve primary mesenteries, of which two opposite 

 pairs are directives, is fairly certain, in spite of the simple 

 character of the remaining mesenteries. But the structure of 

 its retractor muscles is so unlike that known in any other 

 Anthozoau, that it must stand as the type of a new family, the 

 Octiueonidse, characterised by a specialisation of the retractor 

 muscle-fibres into a muscle separate, or nearly so, from the 

 mesentery ; probably also by other of its characters. 



The next question for discussion is — Does the structure 

 of Octineon yield any indication of its phylogenetic history, 

 or of that of other Anthozoa ? Two important papers^ have 

 appeared of late on the interrelations of the various groups of 

 Anthozoa, both possessed by a central idea ; namely, that the 

 Anthozoa having a larva with eight mesenteries, the muscles 

 of which are in many cases arranged as in Edwardsia, the 

 Edwardsise are the modern representatives of the starting- 

 point from which all our present groups originally diverged. 

 The premiss of the argument, however, has been contradicted 

 by van Beneden^ so far as relates to the Cerianthese ; it is 

 true of about half of the recorded cases of Hexactinian deve- 

 lopment, and is an inference only in the Zoantheae. 



While admiring the brilliant ingenuity with which McMur- 

 rich and Boveri have worked out their suggestive theory, I 

 must say that its weakness seems to lie in the absence of 

 evidence as to what constitutes homology among mesenteries 

 and siphonoglyphs, on which it is mainly based. 



In the case of the Hexactiuise, with which we are alone con- 

 cerned at the moment, there may be an homology among the 

 first six pairs of mesenteries when two of these are directives; 

 at any rate, there exists no present reason to deny it to them. 

 But the stage with twelve mesenteries is preceded by a stage 

 with eight, a resting stage in the development of some dura- 



^ Boveri, "Ueber Entwicklung und Verwandschaftsbezieliungen der 

 Aktinien," 'Zeit. wiss. Zool.,' xlix, 461. McMukrich, "Contributions on 

 the Morphology of the Actinozoa : iii, The Phylogeny of the Actinozoa," 

 'Journ. Morph.,' v, 125. 



^ Van Beneden, " Becherches sur le Developpement des Araelinactis," 

 'Arch. Biol.,' xi, 115. 



