In Actinians the epithelio-muscular cells of the endoderm are crowded with yellow spherical bodies, which are unicellular plants or Algae, living symbiotically in the tissues of the zooid.
For further details of the structure of Actinians, the reader should consult the work of 0.
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 arrange 4 4 ¢ 2 4 FIG.
There are, in addition to these groups, several genera of Actinians whose mesenterial arrangement differs from the normal type.
All recent corals, as has already been said, conform so closely to the anatomy of normal Actinians that they cannot be classified apart from them, except that they are distinguished by the possession of a calcareous skeleton.
(For an account of coral formations see Coral-Reefs.) In the present state of our knowledge the Zoantharia in which a primary cycle of six couples of mesenteries is (or may be inferred to be) completed by the addition of two pairs to the eight Edwardsian mesenteries, and succeeding cycles are formed in the exocoeles of the pre-existing mesenterial cycles, may be classed in an order Actinjidea, and this may be divided into the suborders Malacactiniae, comprising the soft-bodied Actinians, such as Actinia, Sagartia, Bunodes, &c., and the Scleractiniae, comprising the corals.