mpressed laterally, and is furnished with two longitudinal grooves,
a sulcus and a sulculus. The arrangement of the muscle-banners on the
mesenteries is characteristic. On six of the mesenteries the
muscle-banners have the same position as in the Alcyonaria, namely, on
the sulcar faces; but in the two remaining mesenteries, namely, those
which are attached on either side of the sulcus, the muscle-banners
are on the opposite or sulcular faces. It is not known whether all the
eight mesenteries of _Edwardsia_ are developed simultaneously or not,
but in the youngest form which has been studied all the eight
mesenteries were present, but only two of them, namely the
sulco-laterals, bore mesenterial filaments, and so it is presumed that
they are the first pair to be developed. In the common sea-anemone,
_Actinia equina_ (which has already been quoted as a type of Anthozoan
structure), the mesenteries are numerous and are arranged in cycles.
The mesenteries of the first cycle are complete (i.e. are attached to
the stomodaeum), are twelve in number, and arranged in couples,
distinguishable by the position of the muscle-banners. In the four
couples of mesenteries which are attached to the sides of the
elongated stomodaeum the muscle-banners of each couple are turned
towards one another, but in the sulcar and sulcular couples, known as
the directive mesenteries, the muscle-banners are on the outer faces
of the mesenteries, and so are turned away from one another (see fig.
10, C). The space enclosed between two mesenteries of the same couple
is called an _entocoele_; the space enclosed between two mesenteries
of adjacent couples is called an _exocoele_. The second cycle of
mesenteries consists of six couples, each formed in an exocoele of the
primary cycle, and in each couple the muscle-banners are _vis-a-vis_.
The third cycle comprises twelve couples, each formed in an exocoele
between the primary and secondary couples and so on, it being a
general rule (subject, however, to exceptions) that new mesenterial
couples are always formed in the exocoeles, and not in the entocoeles.

[Illustration: FIG. 11.--A, Diagram showing the sequence of
mesenterial development in an Actinian. B, Diagrammatic transverse
section of _Gonactinia prolifera_.]

While the mesenterial couples belonging to the second and each
successive cycle are formed simultaneously, those of the firs