Haddon and Shackleton — A Revision of the British Actinice. 613 



The mesenteries whicli yield such a valuable aid to classification in the 

 Actinise generally are arranged in this group in a very uniform manner. 



In the first part of this revision (1889, p. 343), a short account is given of the 

 history of the elucidation of the arrangement of the mesenteries in the Zoanthese. 

 Since this was written the second jDart of Hertwig's " Challenger " Report has been 

 published, without, however, adding anything to Erdmann's account. M^Murrich 

 has also written two valuable Papers (1889 and 1889 a), but no new type of mesen- 

 teric arrangement has been described beyond those first pointed out by Hertwig 

 (1883), and properly described by Erdmann. 



It is necessary to have a recognized system of terminology in order to describe 

 the arrangement of the mesenteries in the Actinise ; and it is advisable to have 

 such a terminology as is applicable to the whole of the Actinozoa. One of us has 

 already (1889) proposed the adoption of certain terms, and the abolition of others 

 which have not a precise meaning — as, for example, such words as " dorsal" and 

 "ventral," these latter were replaced by "sulcular" and " sulcar," respectively. 

 When only one axial oesophageal groove is present, it is usually (? always) the 

 sulcar. In the same Paper attention was called to the value of the order of the 

 appearance of the mesenteries in young Actinise, as suggesting the affinities of 

 different groups of sea-anemones. The following diagram illustrates the pro- 

 posed method of naming the mesenteries and chambers at a stage when twelve 

 mesenteries have made their appearance : — 



^>s^ Sulcular directives. 



Sulcular endoooele .^ 1^^^^^ 



Sulcular exoccele y^. 3 E ^^ 



X 3 A ^ ^^ Sulculo-sulcular lateral. 



Sulculo-lateral endoccele ..-/_ ^Jj. 1/ if ^J^ ^ - Sulco-siUcular lateral. 



Lateral exocoele I... J L I 



Sulco-lateral endoccele. 



Sulcar exocosle 



Sulcar endoccele 



V**^^ II li >iu^^^*T Sulculo- sulcar lateral. 



^^^ a E ^^^ Sulco-sulcar lateral. 



^"VTT. Sulcar directives. 



Examples. — Edwardsia has a pair of sulcular and a pair of sulcar directives ; a 

 sulculo-sulcular lateral mesentery, and a sulculo-sulcar lateral mesentery on each 

 side, all of which are perfect. 



The larval form of Zoanthus has a pair of sulcular imperfect directives and a 

 pair of sulcar perfect directives ; a sulculo-sulcular lateral perfect mesentery, a 

 sulco- sulcular lateral imperfect mesentery, a sulculo-sulcar lateral perfect mesen- 

 tery, and a sulco-sulcar lateral imperfect mesentery on each side. 



