Fig. 11.—Corals. A, Acanthoporia horrida. B, Meandrina strigosa. C, Madrepora divaricata. D, Fungia papillosa. E, Red Coral, Corallium rubrum. F, Stylaster sanguineus.
The above diagram shows examples of the Anthozoa. [Fig. 10] is Gorgonia, the Sea-Fan; while [Fig. 11] represents corals of six different kinds.
Besides the two great groups we have named, the Hydra-like animals and the Sea-Anemone-like animals, the Cœlenterata contain a third group, the Ctenophora, or Comb-bearers, so called on account of their possessing bands of cilia, fancifully compared to the teeth of a comb. At first sight most of them somewhat resemble jelly-fishes, being transparent forms swimming near the surface of the sea. They are carnivorous, and some of them highly phosphorescent at night. The gastric cavity is divided up into branches. The representatives of the Ctenophores, most often seen on our own coasts, are small rounded forms.
Two remarks must be added before quitting the subject of the Cœlenterata.
Firstly, the description of them as two-layered Animals is one that only applies typically and to the simpler forms. In others, such as the jelly-fishes, there is an intermediate layer of jelly, which appears to acquire a cellular structure by the immigration of cells derived from the primary layers. Thus we see, within the group of the Cœlenterata, the gradual establishment of that third body-layer, which is found in all animals of higher structure. Scarcely indicated in Hydra, as a faint trace of a boundary-line (lamella) between the ectoderm and endoderm, it attains a good thickness in the Jelly-fish and Ctenophora. In animals of higher structure the third body-layer, being now fully established, is cellular from its beginning in the embryo; in the Cœlenterata its gradual formation is to be traced.
Secondly, it must be remarked that the colonial structure and the arrangement sometimes concomitant with it of "alternation of generations," is by no means confined to the Cœlenterata. Both are seen in other forms of life, in which the units, or zooids, differ greatly in structure from those of this group.
TABLE SHOWING THE CLASSIFICATION OF THE CŒLENTERATA
| Grade II. The Two-layered Animals. | CŒLENTERATA. | HYDROZOA, or HYDRA-LIKE ANIMALS. ACTINOZOA, or SEA-ANEMONE-LIKE CTENOPHORA |