Comparative Embryology Volume 1

Comparative Embryology Volume 1

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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1885 edition. Excerpt: ...equivalent to the cement gland in the adult Loxosoma, as the molluscan shell-gland. Lankester's interpretations are very plausible, but at the same time they appear to me to involve considerable difficulties. There is absolutely no evidence amongst the Mollusca of the existence of a primitive longitudinal ciliated ring, such as he supposes to have existed, and Lankester is debarred from regarding the ciliated ring of the Polyzoa as equivalent to the simple velar ring of the Mollusca, because his shell-gland lies in the centre and not as it should do on the posterior side of the ciliated ring. Another difficulty which I find is the invariable ciliation of Lankester's shell-gland--a ciliation which never occurs amongst Mollusca. It appears to me that a more satisfactory comparison of the larvae of the Polyzoa with those of the Mollusca is obtained by dropping the view that the ciliated disc is the shell-gland, and by regarding the ciliated ring as equivalent to the velum. This mode of comparison has been adopted by Hatschek. The larva ceases however on this view to have any special molluscan characters (except possibly the organ which Lankester has identified as the foot), and only resembles a molluscan larva to the same extent as it does a larva of the Polychaeta. The ciliated disc lies according to this view in the centre of the velar area or prae-oral lobe, and therefore in the situation in which a tuft of cilia is often present in lamellibranchiate and other molluscan larvae, and also in the larvae of most Chaetopoda. It is moreover at this point that the supra-oesophageal ganglion is always formed in the Mollusca and Chaetopoda as a thickening of the epiblast (fig. 134, sg.), so that the thickening of the epiblast in the ciliated disc of more

Product details

  • Paperback | 224 pages
  • 189 x 246 x 12mm | 408g
  • Miami Fl, United States
  • English
  • black & white illustrations
  • 1236504542
  • 9781236504548