38 Proceedings of Royal Society of Edinburgh. [sess. 
the three pairs which I have termed secondary in Antipathidae. 
The shortest mesenteries in Leiopathes are the last of the six pairs 
to he developed in Heliactis. Evidently, then, the mesenteries 
forming a pair are originally opposite mesenteries, and not adjacent 
ones. We thus have in forms with an elongated stomodaeum a true 
bilateral symmetry. The two pairs of directives limit an anterior 
and a posterior unpaired chamber. Between these two the ccelen- 
teron may he imperfectly divided into any number of paired lateral 
chambers. On this interpretation the arrangement in Alcyonaria, 
Edwardsiae, Cerianthidae, Madracis, &c., is also easily understood ; 
all are modifications of one plan. 
In the Hexactiniae the simple bilateralism is masked, but a careful 
study of the order in which the mesenteries are developed shows 
clearly how this is brought about. In all types the mesenteries of 
a pair are originally on opposite sides of the stomodseum. The two 
pairs of “ directives’’ come to be adjacent mesenteries, for the reason 
that no new mesenteries are ever formed between them, and with a 
further development of mesenteries they come to be pushed closer 
together. As is clearly seen from Hertwig’s figures of the embryonic 
condition in Peachia , the other so-called “ pairs ” of primary mesen- 
teries are not pairs developmentally, as they consist of mesenteries 
of different ages. They are called pairs because they are arranged 
in couples, having the retractor muscles on their inner surfaces. In 
Hexactiniae the further increase in the number of mesenteries takes 
place in a modified way. Buds appear which are on opposite sides 
of the stomodaeum, between existing “pairs,” but instead of giving 
rise to a single mesentery as before, each gives rise to two with the 
retractor muscles on their inner surfaces. 
The general plan of development I consider to be as follows: — 
The mesenteries have a radiate arrangement in forms with a round 
stomodaeum ; this arrangement becomes bilateral by an elongation of 
the stomodaeum in one axis, the sagittal. In this case the anterior 
and posterior pairs (directives) come to consist of adjoining mesen- 
teries, whilst the intermediate pairs consist of opposite mesenteries. 
So long as the folds of the body-wall give rise to only one mesentery 
each, the simple bilateral arrangement is retained, as in Cerianthidae 
(I refer to a bilateral arrangement of parts, irrespective of the out- 
line of the polyp). In case the mesenterial rudiments give rise 
