. Zoology : for students and general readers . Zoology. 234 ZOOLOGY. articulates, interpolated between the j)enultimate and ter- minal segments of the body. At D, the body is many- jointed, the tentacles well developed, the large temporary bristles have been discarded, and the worm can be identified as a young Polydora. It is j)robable that Polydora is hatched as a trocJiosphere like that of Polyzoa, Brachiopoda and certain mollusks. The young Tereirellides Stroemii, and of Linnhriconereis, are at first trochospheres, i. e., the free-swimming germ is spherical, with a zone of cilia, two eye- s
. Zoology : for students and general readers . Zoology. 234 ZOOLOGY. articulates, interpolated between the j)enultimate and ter- minal segments of the body. At D, the body is many- jointed, the tentacles well developed, the large temporary bristles have been discarded, and the worm can be identified as a young Polydora. It is j)robable that Polydora is hatched as a trocJiosphere like that of Polyzoa, Brachiopoda and certain mollusks. The young Tereirellides Stroemii, and of Linnhriconereis, are at first trochospheres, i. e., the free-swimming germ is spherical, with a zone of cilia, two eye- spots, and no bristles. Thus the earliest stages of Polyzoa, Brachiopoda, LameUibrancMata, Oastro- poda, and even of a Cephalopod (Fig. 220), Nemer- tina, and Annelides are almost identical. Farther along in their developmental history, the cej)ha- lula of the Annelides (Fig. 153, A, B, and 155), is like that of certain Echinoderms (Fig. 155), Geypihyrea, Polyzoa, Brachiopoda, and Mollusca. It may here be observed that the free-swimming larvas of these types of invertebrate animals are the young of more or less seden- Larva of Phyllo(k>ce.— After A. Ag- Fig. 155.—Cephalula stage of Echinoderms ana Worms, lateral view. A, Holo- thunan, B, Star-fish, C\ Z>, of Annelides. 0, mouth ; i. stomach ; «, vent; v^ prjeoral ciliated band, in B, C, D, independent; in A surrounding an oral region.—From Gegenbaur. tary parents. In this way the species becomes widely dis- tributed through the action of the marine currents, and too dose in-and-in breeding is prevented. Certain Annelides sometimes multiply by self-division, the process being called strolilation. This is commonly observed. 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 Packard, A. S. (Alpheus Spring), 1839-1905. New York : Henry Holt
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectzoology, bookyear1879
