f six
couples of mesenteries is always formed, and in all the cases which
have been examined the course of development described above is
followed. There are, however, several groups of Zoantharia in which
the mesenterial arrangement of the adult differs widely from that just
described. But it is possible to refer all these cases with more or
less certainty to the Edwardsian type.

The order ZOANTHIDEA comprises a number of soft-bodied Zoantharians
generally encrusted with sand. Externally they resemble ordinary
sea-anemones, but there is only one ciliated groove, the sulcus, in
the stomodaeum, and the mesenteries are arranged on a peculiar
pattern. The first twelve mesenteries are disposed in couples, and do
not differ from those of Actinia except in size. The mesenterial pairs
I, II and III are attached to the stomodaeum, and are called
macromesenteries (fig. 12, B), but IV, V and VI are much shorter, and
are called micromesenteries. The subsequent development is peculiar to
the group. New mesenteries are formed only in the sulco-lateral
exocoeles. They are formed in couples, each couple consisting of a
macromesentery and a micromesentery, disposed so that the former is
nearest to the sulcar directives. The derivation of the Zoanthidea
from an Edwardsia form is sufficiently obvious.

The order CERIANTHIDEA comprises a few soft-bodied Zoantharians with
rounded aboral extremities pierced by pores. They have two circlets of
tentacles, a labial and a marginal, and there is only one ciliated
groove in the stomodaeum, which appears to be the sulculus. The
mesenteries are numerous, and the longitudinal muscles, though
distinguishable, are so feebly developed that there are no
muscle-banners. The larval forms of the type genus _Cerianthus_ float
freely in the sea, and were once considered to belong to a separate
genus, _Arachnactis_. In this larva four pairs of mesenteries having
the typical Edwardsian arrangement are developed, but the fifth and
sixth pairs, instead of forming couples with the first and second,
arise in the sulcar chamber, the fifth pair inside the fourth, and the
sixth pair inside the fifth. New mesenteries are continually added in
the sulcar chamber, the seventh pair within the sixth, the eighth pair
within the seventh, and so on (fig. 13). In the Cerianthidea, as in
the Zoanthidea, much as the adult arrangement of mesenteries differs