t cycle
are formed in successive pairs, each member of a pair being placed on
opposite sides of the stomodaeum. Hence the arrangement in six couples
is a secondary and not a primary feature. In most Actinians the
mesenteries appear in the following order:--At the time when the
stomodaeum is formed, a single pair of mesenteries, marked I, I in the
diagram (fig. 11, A), makes its appearance, dividing the coelenteric
cavity into a smaller sulcar and a large sulcular chamber. The
muscle-banners of this pair are placed on the sulcar faces of the
mesenteries. Next, a pair of mesenteries, marked II, II in the
diagram, is developed in the sulcular chamber, its muscle-banners
facing the same way as those of I, I. The third pair is formed in the
sulcar chamber, in close connexion with the sulcus, and in this case
the muscle-banners are on the _sulcular_ faces. The fourth pair,
having its muscle-banners on the sulcar faces, is developed at the
opposite extremity of the stomodaeum in close connexion with the
sulculus. There are now eight mesenteries present, having exactly the
same arrangement as in Edwardsia. A pause in the development follows,
during which no new mesenteries are formed, and then the six-rayed
symmetry characteristic of a normal Actinian zooid is completed by the
formation of the mesenteries V, V in the lateral chambers, and VI, VI
in the sulco-lateral chambers, their muscle-banners being so disposed
that they form couples respectively with II, II and I, I. In _Actinia
equina_ the Edwardsia stage is arrived at somewhat differently. The
mesenteries second in order of formation form the sulcular directives,
those fourth in order of formation form with the fifth the
sulculo-lateral couples of the adult.

[Illustration: FIG. 12.

A, Zoanthid colony, showing the expanded zooids.

B, Diagram showing the arrangement of mesenteries in a young Zoanthid.

C, Diagram showing the arrangement of mesenteries in an adult
Zoanthid. 1, 2, 3, 4, Edwardsian mesenteries.]

As far as the anatomy of the zooid is concerned, the majority of the
stony or madreporarian corals agree exactly with the soft-bodied
Actinians, such as _Actinia equina_, both in the number and
arrangement of the adult mesenteries and in the order of development
of the first cycle. The few exceptions will be dealt with later, but
it may be stated here that even in these the first cycle o