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 218 CCELEKTEEATA. 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 fiss
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 218 CCELEKTEEATA. 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 tho production of germs or gemmules, but also by the formation of ova and sperm capsules. The gemmules are in the fresh-water SpongiMa masses of cells which are surrounded by a firm shell composed of silicious structm-es (ampkidiscs], 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 arnreboid 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 SpongiMa, but more recently has been shown to exist in almost every group of Sponges. In most cases the ova and spermatozoa 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 arc derived fro
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