. A manual of zoology. Zoology. 274 COiLlIELMINTHES disappear in the adult. In many worms the septa between the somites also breaks down and the ccelomic cavities miite into one. The nephridia also arise independently of the protonephridial system, which is often called head kidney because the cliiel part of the trochophore forms the head of the adult. B. kn Fig. 257.—. 1, larva o£ Polygonlius; P, same changing to segmented worm (after Hatschek). a, anus; kn, excretory organ; mcs, seginentcJ mesoilerm. The land and fresh-water annelids develop directlv, but the embryos pos- sess a reminiscence
. A manual of zoology. Zoology. 274 COiLlIELMINTHES disappear in the adult. In many worms the septa between the somites also breaks down and the ccelomic cavities miite into one. The nephridia also arise independently of the protonephridial system, which is often called head kidney because the cliiel part of the trochophore forms the head of the adult. B. kn Fig. 257.—. 1, larva o£ Polygonlius; P, same changing to segmented worm (after Hatschek). a, anus; kn, excretory organ; mcs, seginentcJ mesoilerm. The land and fresh-water annelids develop directlv, but the embryos pos- sess a reminiscence of a larva in that the head lobes are very apparent and contain protonephridia, which leads to the conclusion that these animals earlier had a metamorphosis. From the resemblance of the trochophore to the Rolifera the farther conclusion is drawn that the annelids ha\e descended from rotifer-like ancestors, the body ca\ily, nelihridia, blood-vessels, and ventral nerve chain being new formations. Besides sexual reproduction maitv fresh-water and marine species reproduce asc-xually, tliis being possible from the great honiotroray of the segmentation. By rapiil growth at the liinder end as well as at a more anterior budding zone numerous somites are formed, wltich separate in groups from the piareirt to form voutrg worms. In some cases the forma- tion of new somites may take place more rapidly than the separation, the result being chains of worms (tig. 25S) wliich in some instances branch. T!y a combination of sexual and asexual reproduction a tyjilcal alternation of generations occurs, the origin of which receives light from the following facts:. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Hertwig, Richard, 1850-1937; Kingsley, J. S. (John Sterling), 1854-1929, ed. and tr. New York, H. Holt
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectzoology, bookyear1912
