. Elementary text-book of zoology. Zoology. 220 C(ELENTEEATA. After the completion of the tolerably regular segmentation (fig. 1G6, a—c), /S'i/con (Syca/ndroi) ra/phanus passes through a. blastosphere stage, during 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 (fig. 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
. Elementary text-book of zoology. Zoology. 220 C(ELENTEEATA. After the completion of the tolerably regular segmentation (fig. 1G6, a—c), /S'i/con (Syca/ndroi) ra/phanus passes through a. blastosphere stage, during 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 (fig. 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 mesoderm are derived from the dark granular cells, the ciliated cells give rise to the entoderm 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. 167.—Young Si/con (after Fr. E. Schulze). O, Osculum or exhalent 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 Myxos^>ongice and ChalinecK, 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 glassy 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 Hexactinellidtu especially are met with in the most ancient Silurian. Please note that these images are extracted from scanned page images that may have be
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
