from that of Actinia, the derivation from an Edwardsia stock is
obvious.

[Illustration: FIG. 13.

A, _Cerianthus solitarius_ (after A. Andres).

B, Transverse section of the stomodaeum, showing the sulculus, sl, and
the arrangement of the mesenteries.

C, Oral aspect of _Arachnactis brachiolata_, the larva of
_Cerianthus_, with seven tentacles.

D, Transverse section of an older larva. The numerals indicate the
order of development of the mesenteries.]

The order ANTIPATHIDEA is a well-defined group whose affinities are
more obscure. The type form, _Antipathes dichotoma_ (fig. 14), forms
arborescent colonies consisting of numerous zooids arranged in a
single series along one surface of a branched horny axis. Each zooid
has six tentacles; the stomodaeum is elongate, but the sulcus and
sulculus are very feebly represented. There are ten mesenteries in
which the musculature is so little developed as to be almost
indistinguishable. The sulcar and sulcular pairs of mesenteries are
short, the sulco-lateral and sulculo-lateral pairs are a little
longer, but the two transverse are very large and are the only
mesenteries which bear gonads. As the development of the Antipathidea
is unknown, it is impossible to say what is the sequence of the
mesenterial development, but in _Leiopathes glaberrima_, a genus with
twelve mesenteries, there are distinct indications of an Edwardsia
stage.

[Illustration: FIG. 14.

A, Portion of a colony of _Antipathes dichotoma_.

B, Single zooid and axis of the same magnified. m, Mouth; mf
mesenterial filament; ax, axis.

C, Transverse section through the oral cone of _Antipathella minor_,
st, Stomodaeum; ov, ovary.]

There are, in addition to these groups, several genera of Actinians
whose mesenterial arrangement differs from the normal type. Of these
perhaps the most interesting is _Gonactinia prolifera_ (fig. 11, B),
with eight macromesenteries arranged on the Edwardsian plan. Two pairs
of micromesenteries form couples with the first and second Edwardsian
pairs, and in addition there is a couple of micromesenteries in each
of the sulculo-lateral exocoeles. Only the first and second pairs of
Edwardsian macromesenteries are fertile, i.e. bear gonads.

The remaining forms, the ACTINIIDEA, are divisible into the
Malacactiniae, or soft-bodied sea-anemones, which have already been
described sufficiently