Haddon and Shackleton — A Revision of the British Actinice. 621 



" inclined to believe that it is produced from the endoderm." His reason is that the 

 zooxanthellse, which are found in the mesenteric canal of this species, " force their 

 way into the septal canal, but never into the canals of the wall." According to our 

 experience there are no connexions between the canal-s3^stem of the body-wall and 

 the mesenteric canals in the oesophageal region of the body, although they are 

 numerous lower down. The section Hertwig figures is from the oesophageal region. 



M'^Murrich (1889a, p. 64) finds in Z. sociatus, in "one mesentery, the basal 

 canal communicating with one of the spaces in the mesogloea of the column 

 wall." He adds: "It seems open to question whether the cells of the larger 

 cavities in the mesogloea are not in reality endodermal in their origin." In his 

 subsequently written, but previously published Paper (1889, p. 114), in alluding 

 to the body-wall canals of Z. flos-marinus, he refers the reader to his description 

 of Isaurus \_^^ Ilammillife^'a^'''] tulerculatus for his views as to the origin of these 

 canals, evidently believing that the " endodermal bays " of the latter species give 

 rise to some of the canals, the remainder being similarly derived from the 

 ectodermal bays, and then not unnaturally concludes that the mesenteric canals 

 are connected only with the endodermal " spaces " of the body-wall. We have 

 corroborated M'^Murrich's observation as to the double origin of the canal-system 

 in the body-wall of Isaurus; but we have no reason for supposing that the canals 

 in the mesenteries are without exception of endodermal origin, although some 

 undoubtedly are (PI. lxiii., fig. 5). Our experience leads us to the conclusion that 

 it is necessary to be cautious in arguing as to the origin of these canals from one 

 genus to another. For confirmation of our view, that the mesenteric canals of 

 Zoanthus are of ectodermal origin, we point to the demonstration of this fact for 

 Z. copping eri (PI. LXii., fig. 1). 



In Z. coppingeri the canals form at the base of each mesentery a large sinus, 

 which extends up the mesentery, nearly to the generative region, Avhere it rapidly 

 narrows, and extends right up the mesentery as the "basal canal." In Z.jukesii 

 and in Z. macgillivrayi the mesenteric canals are present, though not foi'ming 

 extensive sinuses (PI. lxii., fig. 2). 



In I. asymmetricus the canal system appears in sections as if broken up into 

 lacunae ; but it may really form a continuous system of anastomosing canals. 



In Gemmaria rusei (1889, p. 125) and G. isolata (1889 A, p. 66) the mesenteric 

 canal, according to M^Murrich, has the character of a basal canal. We find the 

 same in our two species. Sinuses, extending throughout the whole length of the 

 mesenteries, occur in the three species of Palythoa we have examined. So far as 

 our experience goes canals are absent from the mesenteries of Epizoanthus, and the 

 same obtains for all the species of Parazoanthus except P. anguicoma and P. dixoni, 

 in which species we have found sinuses in the bases of the mesenteries which have 

 a slight vertical extension. 



