COMPARATIVE ANATOMY OF SOME BRITISH ACTINIA. 533 



S. sfhyrodeta resembles S. mihnani (H. and S.) in the following 

 respects : — 



1. The mesogloea is not thick. 



2. The mesenterial filaments are weak. 



3. The parieto-basilar muscle is very feeble ; only a few small folds can 



be seen on the side of the mesentery bearing the retractor muscle. 



With regard to character and shape, the sphincter of S. sfhyrodeta 

 resembles the sphincter of S. carlgreni, but the arrangement of the 

 mesenteries is quite difEerent in the three species. 



S. mihnani has 48 mesenteries, among which a sex radiate symmetry 



is maintained thus : 12+12+24. There are two pairs of directives 



arranged thus : — 



D. 4, D. 6. 



In S. carlgreni the mesenterial formula is 6+6+12+24=48. 



In S. Sfhyrodeta there is one oesophageal groove and one pair of direc- 

 tives. 



The mesenteries are irregularly arranged and I am unable to make a 

 mesenterial formula. Since the tentacles are arranged on an octamerous 

 plan we should expect the mesenteries to be arranged in the same way. 

 In a young specimen I counted 20 pairs and one odd mesentery. Some 

 of these were complete and bore reproductive organs, the others were 

 rudimentary. In an older specimen there were 48 pairs of mesenteries 

 arranged, probably, in four cycles. 



The mesenteries of this species are stronger and the retractor muscle 

 forms a definite cushion with branched muscle folds. 



