126 THE VOYAGE OF H.M.S. CHALLENGER. 
Development of Alcyonarians and Zoantharians compared.—It seems extremely 
difficult to reconcile the extraordinary succession of the mesenteries in the development 
of the Zoantharians, discovered by Lacaze-Duthiers, with the facts presented by Alcyona- 
rians. Did the development of the eight mesenteries of Alcyonaria correspond with that 
of the first eight mesenteries formed in Actiniadee, the first mesenteries formed would be 
either the lateral dorsal or lateral ventral; but these are those which are most rudimen- 
tary in the zooids of Sarcophyton. Moreover, the mesenterial filaments of the two lateral 
pairs of septa are in the development of Actiniade the first to appear, and not the dorsal, 
which are longest in the Aleyonarian polyps and most persistent in the zooids. 
Apparently, however, development in Alcyonarians follows a different course. 
In Halysceptrum, the development of which has been examined by Kolliker, the 
cight mesenteries appear from the very first. In Aalliphobe (Busch), one of the 
Edwardsiz, according to Metschnikoff, the larva has, in its earliest stage, eight tentacles 
and two mesenterial filaments. 
