140 McMURRICH. [Vol. V. 



Instead of speaking of a suppression of mesenteries in this case, 

 it may be better, perhaps, to say that the octameral condition 

 has been brought about by the acceleration of the development 

 of a single pair of paired mesenteries, so that they become con- 

 temporaneous with the two Halcampa mesenteries (V and VI). 

 These contemporaneous mesenteries becoming perfect at the 

 same time, we would get an octamerous form ; and the ordinary 

 radial development of the succeeding mesenteries then coming 

 in would bring about the arrangement found in Aiptasia annulata 

 and Sideractis. 



IV. ZoanthEjE. 



Our knowledge of the structure of the adult Zoantheae has 

 been largely added to in recent years, more especially by Von 

 Koch ('So), 1 R. Hertwig ('82), and Erdmann ('85), and from 

 the information which we possess it is easy to determine, as 

 Erdmann has done, the relation in which the members of this 

 group stand to the Hexactiniae. The five pairs of dorsal mes- 

 enteries, together with the ventral directives, seem to represent 

 the twelve mesenteries of the Halcampa stage, later formed 

 mesenteries developing always in the exoccels, on either side of 

 the ventral directives, instead of, as in the Hexactiniae, in all 

 the exoccels present in the stage with twelve mesenteries. 



Our knowledge of larval forms is, however, as yet exceedingly 

 scanty. Semper (67), some years ago, described two peculiar 

 pelagic larvae from the region of the Cape of Good Hope, which 

 apparently were stages in the development of an Anthozoan. 

 One of these, usually known as Semper's larva, is a cylindrical 

 organism, with an opening at each end, and is characterized by 

 a longitudinal band of iridescent cilia running from one ex- 

 tremity to the other, along one surface. The other form was 

 more globular in shape, though capable of elongation, and dif- 

 fered in possessing a circular ring of cilia. Almost nothing 

 was ascertained regarding the internal structure of these forms, 

 so that it was impossible to determine to what group of Antho- 

 zoa they belonged, or even to be certain that they really were 

 Anthozoan. Very recently, however, Van Beneden ('90) has 



1 I regret exceedingly that in a previous paper ('89) I failed, through an oversight, 

 to ascribe the discovery of the true arrangement of the mesenteries of the Zoantheae 

 to Von Koch. 



