Elementary text-book of zoology, tr Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote elementarytextbo01clau Year: 1892-1893 220 ( After the completion of the tolerably regular segmentation (fig. 1G6, a—c), Sycon (Sycandra') raphanus passes through a blastosphere stage, dining which the greater half of the ovum consists of clear cylindrical cells, and the smaller half at the still open pole of large dark granular cells (tig. 166, d). The cylindrical cells of the larger half develop cilia, and the embyro passes out of the body ca
Elementary text-book of zoology, tr Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote elementarytextbo01clau Year: 1892-1893 220 ( After the completion of the tolerably regular segmentation (fig. 1G6, a—c), Sycon (Sycandra') raphanus passes through a blastosphere stage, dining which the greater half of the ovum consists of clear cylindrical cells, and the smaller half at the still open pole of large dark granular cells (tig. 166, d). The cylindrical cells of the larger half develop cilia, and the embyro passes out of the body cavity and becomes a free-swimming larva, which attaches itself and alters its shape in such a manner that the dark cells grow over the ciliated portion of the globe, which is meanwhile invaginating. The ecto- derm and mesoderni are derived from the dark granular cells, arid the ciliated cells give rise to the entoderna of the gastric cavity. Later on the body of the sponge be- comes cylindrical, the osculum makes its appearance, and calcareous needles appear in the wall, which becomes pierced by pores (fig. 167). With the excep- tion of Spongilla, the sponges are FIG. 1G7.—Young Sycon (after Fr. E. Scliulze). O, Osculum or exlialent aperture; P, pores of the wall. marine, and are met with under very different con- ditions, and covering a wide area of distribution. The horny sponges live in shallow seas, as also the Myxospongice and Cficdinece, or siliciceratous Sponges; while the Hexactinellidce inhabit very considerable depths. Petrified remains of sponges are also found preserved in various formations, for instance in the chalk; and these remains differ much from the greater number of those living. On the other hand, the hyaline sponges of the deep sea agree so fully with the ancient forms that they seem to be the direct descendants of the latter. Finally, many of the principal groups extend back into the palaeozoic age, in which LithistidcE and Hexactinellidce especial
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