. Elementary text-book of zoology. Zoology. 218 CCELENTEBATA. In this case the canal system, in which the modifications before described for each individual Sponge are repeated, becomes more complex, partly through the formation of anastomoses, and partly because irregular gaps and winding passages make their appearance between the fused branches of the stock and form spaces which lead into the ciliated cavities. Reproduction takes* place mainly asexually by fission and the production of germs or gemmules, but also by the formation of ova and sperm capsules. The gemmules are in the fresh-water
. Elementary text-book of zoology. Zoology. 218 CCELENTEBATA. In this case the canal system, in which the modifications before described for each individual Sponge are repeated, becomes more complex, partly through the formation of anastomoses, and partly because irregular gaps and winding passages make their appearance between the fused branches of the stock and form spaces which lead into the ciliated cavities. Reproduction takes* place mainly asexually by fission and the production of germs or gemmules, but also by the formation of ova and sperm capsules. The gemmules are in the fresh-water Spongilla masses of cells which are surrounded by a firm, shell composed of silicious structures (amphidiscs), and, like encysted Protozoa, pass through a long period of rest and inac- tivity. After the expiration of the cold and sterile season of the year, the contents pass out of the opening of the capsule and gene- rally surround the latter, and with increasing growth become differentiated into amoeboid cells and all the essential parts of a new small sponge body. Multiplication by means of gemmules is also common among the marine Sponges. The gemmules take their origin under certain conditions as small globules surrounded by a membrane. The contents are essentially formed of sponge cells and spicules, and, after a longer or shorter period of inactivity, reach the exterior by the rupture of the membrane. Sexual reproduction was first demonstrated with certainty by Lieberkiihn for Spongilla, but more recently has been shown to exist in almost every group of Sponges. In most fa Qya ^ spevmatozoa seem to reach maturity at different times in the same Sponge. The spermatozoa are needle-shaped, and lie in small spaces lined with cells. The ova, like the mother cells of the spermatozoa, are modified cells of the parenchyma, and are derived from cells of the same tissue layer (mesoderm) in which the needles and skeletal structures take their origin. The ova are naked amoeboid cells
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
