THE ANTHOZOA 53 



terial spaces, and that, in addition, there is a median intermesen- 

 terial space on the sulcar side as yet unprovided with a tentacle. 

 The intermesenterial spaces are formed by ten mesenteries, whose 

 developmental sequence is expressed by the numbers 1,1; 2, 2 ; 



3, 3; 4, 4; 5, 5. Thus the larval Cerianthus passes through a stage 

 with eight mesenteries, and these are developed in the same 

 sequence as the first eight mesenteries of Halcampa and Rhodactis. 

 But in the absence of muscle banners it can hardly be called 

 an Edwardsia stage. The further course of development differs 

 from anything else that has been described. New mesenteries are 

 always developed in the sulcar chamber between the previously 

 existing sulcar mesenteries. Thus 5, 5, are developed between 



4, 4 ; 6, 6, will be developed between 5, 5, and so on. It 

 results that Cerianthus is strictly bilaterally symmetrical, and 

 that the members of a mesenterial pair are not contiguous, but 

 are to be found on opposite sides of the stomodaeum. As the 

 sequence of the first four pairs of mesenteries is identical with 

 that of Halcampa, etc., it has been held by Boveri and M'Murrich 

 that the Cerianthidae are derived from an Edwardsian stock. But 

 the presence of longitudinal parietal ectodermic muscles must be 

 held to separate the Cerianthidae from the Edwardsia stock, and, 

 as in the case of the Zoantheae, they must be regarded as having 

 diverged from a common ancestor of all the Zoantharia. 



The Antipathidea form a well-defined group, whose relation- 

 ships are very obscure. The type form, Antipathes dichotomy 

 forms arborescent colonies, consisting of numerous zooids arranged 

 in a single series along one surface of a branched horny axis. 

 The axis is enclosed by the soft tissues, and is surrounded by a 

 special epithelium, which in all probability is of ectodermic origin. 

 Only the number and arrangement of the mesenteries will be con- 

 sidered in this place, further details being postponed. In the Anti- 

 patharian zooid the peristome forms a prominent oral cone, on the 

 summit of which the mouth is placed. It is surrounded by six 

 tentacles, usually simple and non- retractile, but branched and 

 retractile in the family Dendrobrachiidae. The stomodaeum is 

 strongly compressed, and the zooids are so arranged on the axis 

 that the long axis of the stomodaeum is at right angles to the axis 

 of the colony. An ill-defined sulcus and a sulculus are present, 

 and the tentacles corresponding to the sulcar and sulcular inter- 

 mesenterial chambers are longer than the rest. In most of the 

 genera there are ten mesenteries, which do not bear muscle 

 banners. The genus Leiopathes is an exception, having twelve 

 mesenteries. Where ten mesenteries are present, they have the 

 arrangement shown in Fig. XXVI. 2. 



The sulcar and asulcar mesenterial pairs are short ; the sulco- 

 lateral and sulculo-lateral pairs are somewhat longer, but the 



