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 Coelenterata, 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 Coelenterata 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 Coelenterata. 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 COELENTERATA { HYDROZOA, or { HYDRA-LIKE { ANIMALS. =Grade II.= { { THE TWO-LAYERED =COELENTERATA.= { ACTINOZOA, or ANIMALS. { SEA-ANEMONE-LIKE { ANIMALS. { { { CTENOPHORA. CHAPTER VI THE SPONGES