REPORT ON THE ANTIPATHARIA. 53 



end of the long axis of the stomodseum. It now becomes necessary to enquire whether 

 the mesenteries of Antipathidse may not be arranged in pairs, and whether, if this be the 

 case, the arrangement corresponds with that in other Zoantharia. In the Hexactinise all 

 the mesenteries are usually supposed to be arranged in pairs, which consist of adjoining 

 mesenteries. The section of the ccelenteron situate between a pair of mesenteries 

 is termed " intraseptal," that between adjoining pairs "interseptal." Two pairs are 

 known as " directives ; " these are situated in the sagittal axis, and usually terminate 

 the long axis of the mouth. On these the retractor muscles occupy the inter- 

 septal surfaces ; in all other cases, the intraseptal. Thus the position of the retractor 

 muscles enables one to determine the paired arrangement of the mesenteries, and also to 

 decide which are to be considered " directives." Unfortunately in nearly all the Anti- 

 pathidse the muscular system is so feebly developed that I have not yet been able to 

 make out any special collection of muscular fibres on either side of a mesentery. On 

 account of the extremely small size of the zooids of most species, the mesenteries are 

 rarely well developed, and even in such species as Parantipathes larix, where those in 

 the transverse axis are much elongated, there appears no corresponding increase in the 

 importance of their musculature. Thus the sure guide to the paired arrangement in 

 many other Zoantharia is not available here. In this connection it should, however, be 

 remembered that in the Cerianthidse there are apparently no retractor muscles, and the 

 protractors are very feebly developed, added to which the mesenteries are not arranged 

 in pairs on the Hexactinian plan. In the Zoanthidas also the musculature is more 

 rudimentary than in Hexactinise. 



Undoubtedly, the fact that there are twelve mesenteries and six tentacles in Leio- 

 pathes would seem to indicate, on a priori grounds, a paired arrangement of the 

 mesenteries. This seems the more probable when it is remembered that, in all other 

 genera of Antipathidaj of which the zooids are known, there are either ten or six 

 mesenteries, and that in both cases the number might be regarded as directly derivable 

 from the arrangement in Leiopathes. If the twelve mesenteries found in Leiopathes are 

 to be interpreted according to the usual arrangement in Actiniaria and Madreporaria, i.e., 

 on the Hexactinian basis, it is necessary that the mesenteries numbered 1 and 12 

 (fig. 16) should form a pair, as also should numbers 6 and 7. These must then be 

 considered the two pairs of " directives." In this case numbers 2-3, and 4-5 form 

 pairs, and there is a similar paired arrangement on the opposite side of the stomodseum. 

 The effect of this arrangement would be that, of six pairs present in the upper portion of 

 the oral cone, two, the directives, remain intact, two are lost entirely, and the other two 

 pairs become reduced in the lower section of the ccelenteron to a single member each. 

 In this case the sagittal tentacles correspond to intraseptal spaces. The arrangement of 

 the tentacles on each side of the transverse axis is really not the same. Mesenteries 

 3 and 10 are the two which ultimately occupy the transverse axis, and the tentacles to the 



