. Elementary text-book of zoology, general part and special part: protozoa to insecta. Animals. 220 After the completion of the tolerably regular segmentation (fig. 166, a—c), Sijcon {Sijcandra) rcqjhamts passes through a blastosphere stage, duiing 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, cZ). The cylindrical cells of the larger half develop cilia, and the embjT^o passes out of the body cavity and becomes a free-swimming larva, which attaches itself and alters its
. Elementary text-book of zoology, general part and special part: protozoa to insecta. Animals. 220 After the completion of the tolerably regular segmentation (fig. 166, a—c), Sijcon {Sijcandra) rcqjhamts passes through a blastosphere stage, duiing 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, cZ). The cylindrical cells of the larger half develop cilia, and the embjT^o 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, and the ciliated cells give rise to the entodei'm 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 Sjoongilla, the sponges are marine, and are met wdth under very different con- ditions, and covering a wide area of distribution. The horny sponges live in shallow seas, as also the Ilyxosjmnc/icn and Chalinece, or silicicei'atous 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 agi'ee so fully with the ancient forms that they seem to be the dii'ect descendants of the latter. Finally, many of the principal groups extend back into the palaeozoic age, in which Lithistidce and Hexactinellidce especially are met with in the most ancient Silurian. Fig. 1G7.—Young Si/con (after Fr. E. Schulze). O, Osculum or exhalent aperture; P, pores of the Please note that these images ar
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectanimals, bookyear1892
