The new genus Ctenaria recently described by Haeckel, which is intermediate between the Ctenophora and the Medusæ clearly proves that the Ctenophora are more closely related to the Medusæ than to the Actinozoa but their development, especially the presence of a stomodæum, shews that they have affinities (in spite of the rudimentary velum of Ctenaria) with the Acraspedote as well as with the Craspedote Medusæ; and it may be noted that the Acraspeda have undoubted affinities with the Actinozoa. _Summary and general considerations._ Even in the adult condition the lower forms of Coelenterata do not rise in complexity much beyond a typical gastrula. Ontogeny nevertheless brings clearly to light the existence of a larval form--the planula--which recurs with fair constancy amongst all the groups except the Ctenophora. We are probably justified in assuming that the planula is a repetition of a free ancestral form of the Coelenterata. The planula, as it most frequently occurs, is a two-layered ciliated nearly cylindrical organism, with at most a rudimentary digestive cavity hollowed out in the inner layer, and as a rule no mouth. In the outer layer are numerous thread-cells. How many of these characters did the ancestral planula possess? I think it is not unreasonable to assume that the only two characters about which there can be much doubt are the rudimentary condition of the digestive cavity and the absence of a mouth. Paradoxical as it may seem, it appears to me not impossible that the Coelenterata may have had an ancestor in which a digestive tract was physiologically replaced by a solid mass of amoeboid cells. This ancestor was perhaps common to the Turbellarians also. The constant presence of thread-cells in the inner layer of their epiblast fits in with their derivation from a form similar to the planula. While the solid parenchymatous digestive canal of Convoluta and Schizoprora and other forms amongst the Turbellarians, though very probably secondary, may perhaps be explained by such a view of their origin. The planula in its primitive condition is not bilaterally symmetrical, but frequently, as amongst the Actinozoa, it becomes flattened on two sides before undergoing its conversion into the adult form. Perhaps the bilateral form of planula is the starting point both for the Coelenterata and the Turbellaria. In this connection the peculiar unilateral development of a tentacle in Scyphistoma and Actinia should be noted. The planula occurs in the majority of sessile forms of Hydrozoa except the Tubularidæ and Hydra. It is also characteristic of the Trachymedusæ and Siphonophora. Amongst the Acraspeda it is also present, but has an exceptional mode of ontogeny which is discussed in connection with the germinal layers. It is characteristic both of the Octocoralla and Hexacoralla, but is not found in the Ctenophora.