24 PERCY SLADEN TRUST EXPEDITION 



polyp, and "septa" ("cloisons") is restricted to the calcareous radial partitions of the 

 corallites. 



Hollard in 1851 described at length the arrangement of the mesenteries of Actinians 

 in two's, with their respective longitudinal muscles facing each other ; this arrangement 

 has since been recognised in all Hexactinians. 



In polyps of Group I the couples of mesenteries are arranged in alternating cycles, 

 each cycle consisting of six couples of almost uniform width or of a multiple of six, the 

 mesenteries of every succeeding cycle being narrower than those of the preceding one. In 

 the polyps examined thus far, up to four cycles of mesenteries are present, termed 

 respectively primary, secondary, tertiary and quaternary. The so-called "directive" 

 couples* of mesenteries belong to the primary cycle, and are situated at the ends of the 

 longer diameter of the stomodaeum, each enclosing a directive groove ; their arrangement 

 of the mesoglaeal pleats with muscular layers is re versed f. 



For such a disposition of the mesenteries in the adult polyps I prefer to use the term 

 "couples," and "pairs" when referring to their developmental sequence. The word 

 " cycle " is used to include all the couples of mesenteries having more or less the same 

 radial extent, irrespective of their time of development, while "order" has been employed 

 by some authors (Faurot) to denote the mesenteries which have appeared at about 

 the same time to form a circle, e.g. primary order, secondary order, etc. The couples of 

 mesenteries composing the first cycle are usually " complete," i.e. they extend from the 

 column-wall to the stomodaeum ; when one of the mesenteries of a couple does not reach 

 the stomodaeum, the couple is spoken of as " incomplete J." 



In Diploastrea heliopora (Lam.) the primary cycle consists of twelve couples of 

 mesenteries, which may be due to the true secondary couples having grown and met the 

 stomodaeum. Most polyps of Group I possess the full number of secondary couples, 

 viz. six ; in a few polyps the tertiary cycle is also complete with twelve couples, while 

 quaternary couples of mesenteries have only occasionally been met with, viz. in Galaxea 

 fascicularis (Linn.), the cycle itself being quite imperfect. Both Lacaze Duthiers and 

 Faurot have shown that in the Hexactiniae, while the primary couples are formed in pairs 

 on each side of the sagittal plane, the couples of the subsequent cycles are formed in the 

 exocoeles. . 



• In my Group II directive couples of mesenteries are absent, nor is there a sequence of 

 mesenterial cycles as in Group I. The only distinction that could be made between the 

 couples is between those that meet the stomodaeum and those that do not reach it ; the 



* The term "directive" mesenteries (" Richtungssepten ") was first employed by the Hertwigs, as they 

 believed that these two couples might be of use in fixing the orientation of an Actinian polyp ; Schneider and 

 Rotteken (114) had carefully described their arrangement in 1871. Lacaze Duthiers in 1897 (p. 221) doubted 

 the suitability of Hertwigs' terminology, as the two couples in question did not indicate any direction in the 

 polyp. 



t McMurrich (99, p. 1 -32) suggests that the reverse arrangement of the muscles on the directive mesen- 

 teries may have " something to do with the preservation of the lumen of the siphonoglyphe when the lips of the 

 stomodaeum elsewhere are in contact." 



\ Faurot and van Beneden use "complete" and "incomplete," not with reference to the couples or pairs, 

 but to the individual mesenteries that either meet the stomodaeum or do not reach it. The adjectives "perfect " 

 and "imperfect " have also been used in the same sense. 



