ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
607 
to the twelve primary mesenteries, a single mesentery present on each 
side of the dorsal directives. A fourth offset leads to Gonactinia, which 
presents a stage in advance of ScytopJiorus. Further on Oractis is 
reached, in which there are two pairs of imperfect secondary mesenteries. 
All the forms hitherto mentioned are strictly bilateral ; so in reality 
also are the Hexactinian, as may be shown if their structure be regarded 
as a development from the plan seen in Scytophorus, Gonactinia , and 
Oractis. An explanation is offered of the difficulties presented by the 
Halcampae, some of which have, and some of which have not, secondary 
mesenteries ; the latter are looked on as derivates of the former in which 
Tetramerous Form 
Antipatharia ? 
Octamerous Form 
Rugosa? Alcyonaria 
Edwardsiae 
Cerianthet© 
Dodecamerous Form 
Zoanthese 
Protactiniaa 
Hexactiniae (including M&dreporaria) 
there has been an arrest of the development of the secondaries. The 
Peachise appear to have arisen from forms which possessed a complete 
cycle of secondary mesenteries, one pair of which — the dorso-lateraL — 
has been lost 
. Recent observations have taught us that the Madreporaria are 
constructed upon essentially the same plan as the Hexactiniae ; the 
author quotes and accepts Hertwig’s opinion on this point. He sums 
it up in saying that the arrangement of the mesenteries and the order of 
their appearance demonstrate conclusively that the majority, if not all 
of the Hexacoralla are closely related to the Hexactiniae. 
