REPORT ON THE ANTIPATHARIA. 55 



side, and the anterior members of the two " posterior " pairs. This arrangement, dis- 

 regarding for the moment the want of directives, is not without parallel amongst the 

 Actiniaria. G. v. Koch, R. Hertwig, and Erdmann have shown that in the Zoanthidse 

 there is an alternation of macro- and microsepta, which is regular, excepting as regards 

 four pairs. The majority of the pairs of mesenteries consist of a macroseptum and a 

 microseptum, i.e., of one which is complete and another which is incomplete. The 

 macrosepta bear reproductive organs and mesenterial filaments; the microsepta are 

 sterile and end on the oral disc. In the sagittal axis one pair ("ventral") consists of 

 two macrosepta, and corresponds with the single siphonoglyphe. The other pair of 

 " directives " consists of two microsepta. In addition to the directives two other pairs 

 consist of mesenteries of the same type. These are usually situated one on each side, 

 and only a little distance from the small (" dorsal ") directives. They may consist of either 

 micro- or macrosepta, Erdmann explains this peculiar arrangement by supposing that 

 a dorsal and ventral zone of mesenteries exists, and that the two zones approximate either 

 with small (microtype) or large mesenteries (macrotype). According to his investigation, 

 the approach of the two zones is brought about by two mesenteries of the microtype in 

 Zoanthus, Mammilifera and Corticifera. The macrotype arrangement is found in 

 Epizoanthus and Palythoa. The microsepta appear to be rudimentary and not youno- 

 ones, and supposing them to correspond with the imperfect mesenteries oiLeiopathes, there 

 would be a similarity in plan between the arrangement in Leiopathes and those 

 Zoanthidse having the mesenteries arranged on the microtype. 1 According to this view, 

 the tentacles in Leiopathes correspond in the main to interseptal chambers. The 

 intraseptal space between the two pairs of mesenteries in the transverse axis is lost, 

 whilst the other intraseptal areas, in elongate forms, abut on a portion of the lateral 

 body-wall. 



An apparently fatal objection to this explanation consists in the fact that the 

 arrangement of mesenteries in Antipathidae would have no parallel in the Actiniaria 

 or Madreporaria. In these orders the sagittal axis is terminated at each extremity by a 

 pair of " directive " mesenteries and not by two adjoining members of adjacent pairs. 

 The probable absence of a siphonoglyphe may mask the arrangement of mesenteries, and 

 the greatest diameter of the oral aperture may possibly not lie in the true sagittal axis. 

 It may be mentioned that a flattening of the stomodaeum at one, or sometimes at both 

 extremities, has been observed in certain species {e.g., Cirvipathes propinqua), but I am 

 unable at present to determine its significance. Apparently there is no structural 

 difference between this and other portions of the stomodaeum. 



A comparison of the arrangement and comparative development of the mesenteries 

 in Madrepora, Seriatopora, and Leiopathes is of considerable interest. Figures 15, 16, 

 and 17 represent diagrammatically the arrangement of mesenteries in the three genera. 



1 A different interpretation of the arrangement of the mesenteries in Zoanthidse is suggested on p. 59. 



