. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. PRyEORAL LOBE. 269 that of Other Echinoderm larvae, is the well-developed apical plate at its anterior extremity. We may express this in other words by saying that the larva of Antedon possesses a central nervous system at the apex of its praeoral lobe. That the pras- oral lobe in this larva is not sharply marked off from the rest of the body is a detail of no morphological signifi- cance. The apical nervous sys- tem of the Antedon larva was discovered in 1888 by H. Bury, and has been Fig. 128. âFree-swimmin
. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. PRyEORAL LOBE. 269 that of Other Echinoderm larvae, is the well-developed apical plate at its anterior extremity. We may express this in other words by saying that the larva of Antedon possesses a central nervous system at the apex of its praeoral lobe. That the pras- oral lobe in this larva is not sharply marked off from the rest of the body is a detail of no morphological signifi- cance. The apical nervous sys- tem of the Antedon larva was discovered in 1888 by H. Bury, and has been Fig. 128. âFree-swimming larva of 111 1 . Antedon rosacea, from the ventral side. more clearly brought out (After seeliger.) and emphasised in a recent "-t- Epical pole. ciliated bands. f. Fixing disc. v. Vestibulum (so-called work by Dr. Oswald SeELI- iarval mouth, although at this stage A . 1 ⢠. 1 ⢠1 â simply an ectodermic groove). GER. At the point which is '^â ' & > marked externally by the anterior tuft of long cilia in Fig. 129 there is a slight groove in the ectoderm below which nerve-fibres and ganglion-cells can be identified. Seeliger further describes a pair of longitudinal nerves running from the nervous area of the apex along the ventro-lateral margins of the body. As already indicated, the apical plate is, as a general rule, conspicuous by its absence in the typical Echinoderm larva. In the free-swimming larva of Antedon, however, it is emphatically present, although destined to become entirely aborted after the fixation of the larva. In most Invertebrate larvae in which an apical plate is present { the Trochophore-larva of Annelids and Mol- luscs) it becomes, during the metamorphosis, involved in other ectodermic thickenings of the pra;oral lobe, which. 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
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Keywords: ., bookcentury1800, bookdecade1890, bookpublishernewyo, bookyear1894
