612 Haddon and Shacklkton — A Revision of the British Actinia}. 



utilize their results. The laborious monograph of Andres has been in constant 

 requisition. 



It is now our pleasing duty to acknowledge the assistance of many friends. 

 The Rev. Canon. A. M. Norman and Professor W. C. M'Intosh have generously 

 placed the whole of their collections at our service ; and it is due to the considerable 

 number of foreign (Mediterranean and North Atlantic) specimens belonging to the 

 former that we have been enabled to determine several non-British species. The 

 Director of the Marine Biological Laboratory at Plymouth, and Mr. G. Y. Dixon 

 have also supplied us with specimens, as have also our foreign colleagues, 

 Drs. D. C. Danielssen and J. Playfair M'Murrich. Finally, we have to thank 

 Dr. E. Perceval Wright for the loan of books and for ready assistance in the 

 solution of taxonomic and synonymic difficulties. 



GENERAL ACCOUNT OF THE ANATOMY OF THE ZOANTHEJl. 



The main external characters of the Zoanthese have already been given in the 

 Introduction ; and before giving a detailed account of the anatomy of the group it 

 will be necessary to say a few words as to the anatomy of these Actiniae. 



As in other Actinozoa, the body- wall is composed of three layers : the ectoderm, 

 the mesogloea, and the endoderm. There is now no need to adduce arguments in 

 favour of the employment of the second of these terms. 



The mouth leads into a rather short oesophagus or stomatodseum, the walls of 

 which are often thrown into folds ; at one end a distinct and sometimes a very 

 deep groove is present, for which one of us has suggested the name of " sulcus," or 

 sulcar groove. Projecting into the cavity, or coelenteron of the polyp, from its 

 body-wall, are a number of soft j^lates which are known as mesenteries ; sometimes 

 these are called " sarcosepta," and occasionally they are erroneously spoken of as 

 " septa." The employment of the latter term cannot be too strongly deprecated as 

 leading to confusion with the septa, or calcareous radial partitions of the Madre- 

 poraria. 



The mesenteries of the Zoanthese fall under two categories — 



(1) The large mesenteries which extend from the body- wall to the stomato- 

 dseum, and which alone bear mesenterial filaments and gonads. These are the 

 "perfect mesenteries" or "macrosepta" of authors. 



(2) The small mesenteries which extend only slightly from the body- wall into 

 the coelenteron, and which are sterile, and do not bear mesenterial filaments. 

 These are the " imperfect mesenteries" or " microsepta." 



As in most Actiniae a pair of mesenteries occurs at each end of the CESophagus ; 

 these are usually spoken of as the " directives," or " directive mesenteries." 



