36 Proceedings of Royal Society of Edinburgh. [sess. 
longitudinal muscles arranged on different aspects to those of all 
the other mesenteries, whatever may he their number. These are 
known as “ directives,” and are situate one pair at each end of the 
long axis of the mouth ; on these the retractor muscles occupy the 
outer (interseptal) surfaces ; on all the other pairs the retractor 
muscles are placed on the inner (intraseptal) surfaces. 
In a very large number of cases, however, it is impossible to 
explain the arrangement in harmony with this plan. In the whole 
of the Alcyonaria , for instance, there are eight complete mesenteries. 
At one end of the stomodseum a pair of directives occur having the 
same arrangement of muscles as in Hexactiniae ; the other pair of 
directives, however, have the retractor muscles on the inner (intra- 
septal) surfaces; whilst the intermediate mesenteries do not form 
pairs in the usual sense, their muscles not being on adjoining or 
opposite sides, but on the same side. This arrangement may be 
explained simply by stating that all the retractor muscles are 
situated on the abaxial (ventral) surfaces of the mesenteries. In 
Edwardsia and its allies there is an arrangement which does 
not correspond either with the Alcyonarian or Hexactinian type. 
Of the eight mesenteries, two pairs of “ directives” show the 
Hexactinian arrangement ; the other four mesenteries have the 
same arrangement of retractor muscles as in the corresponding 
mesenteries of Alcyonaria. In Cerianthidse the mesenteries are 
numerous — not in multiples of six — and, according to the current 
view, not arranged in pairs. The musculature of the mesenteries is 
rudimentary. 
These four instances will serve to show the variation in the 
arrangement of the mesenteries in some of the leading groups of the 
Anthozoa. Many Madreporaria show the Hexactinian arrangement, 
but others again do not. The Antipatharia are generally supposed 
to be degenerate, and to have lost a considerable number of the 
mesenteries present in their ancestors, but very little is known con- 
cerning the morphology of this group. Dr John Murray has very 
kindly placed the “ Challenger” collection of Antipatharia in my 
hands for identification, and I have now been enabled to make 
sections of twenty-three species of Antipathidae, including the only 
two which had previously been examined by Lacaze Duthiers and 
G. von Koch. A careful comparison of the number and relative 
