(141) F. E. Schulze. "Untersuchung üb. d. Bau, etc. Die Gattung Halisarca." _Zeit. f. wiss. Zool._, Bd. XXVIII. 1877. (142) F. E. Schulze. "Untersuchungen üb. d. Bau, etc. Die Metamorphose von Sycandra raphanus." _Zeit. f. wiss. Zool._, Bd. XXXI. 1878. (143) F. E. Schulze. "Untersuchungen ü. d. Bau, etc. Die Familie Aplysinidæ." _Zeit. f. wiss. Zool._, Bd. XXX. 1878. (144) F. E. Schulze. "Untersuchungen ü. d. Bau, etc. Die Gattung Spongelia." _Zeit. f. wiss. Zool._, Bd. XXXII. 1878. [71] There is a Russian paper by the same author, containing a full account, with clear illustrations, of his observations. CHAPTER VI. COELENTERATA[72]. [72] I. HYDROZOA. 1. Hydromedusæ. {_Hydroidea._ {_Trachymedusæ._ 2. Siphonophora. {_Calycophoridæ._ {_Physophoridæ._ 3. Acraspeda. II. ACTINOZOA. 1. Alcyonaria. (Octocoralla.) 2. Zoantharia. (Hexacoralla.) III. CTENOPHORA. Hydroidea. The most typical mode of development of the Hydroidea is that in which the segmentation leads directly to the formation of a free ciliated two-layered larva, known since Dalyell's observations as a planula. The planula is characteristic of almost all the Hydromedusæ with fixed hydrosomes including the Hydrocoralla (Stylasteridæ and Millepora), the most important exceptions being the genus Tubularia and one or two other genera, and the fresh-water Hydra. In a typical Sertularian the segmentation is approximately regular[73] and ends according to the usual accounts in the formation of a solid spherical mass of cells. A process of delamination now takes place, which leads to the formation of a superficial layer of cubical or pyramidal cells, enclosing a central solid mass of more or less irregularly arranged cells. [73] For a detailed description of the development of a single species the reader referred to Allman's description of Laomedia flexuosa, No. 149, p. 85 _et seq._