. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. PR^OKAL LOBE. 2/1 The very interesting observation has recently been made by MacBride, that the larva of Asterina gibbosa actually undergoes temporary fixation at the beginning of the metamorphosis, the fixation being effected by the prasoral lobe in a manner strikingly similar to that of the larvae of Antedon and of Ciona. In the larva of Antedon the adhering disc, by which the larva eventually fixes itself to some foreign surface, is placed near the front end of the prasoral lobe immediately below the apic

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. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. PR^OKAL LOBE. 2/1 The very interesting observation has recently been made by MacBride, that the larva of Asterina gibbosa actually undergoes temporary fixation at the beginning of the metamorphosis, the fixation being effected by the prasoral lobe in a manner strikingly similar to that of the larvae of Antedon and of Ciona. In the larva of Antedon the adhering disc, by which the larva eventually fixes itself to some foreign surface, is placed near the front end of the prasoral lobe immediately below the apical plate. The central nervous sys- tem of the adult Echinoderm arises in entire indepen- dence of the actual or sup- pressed apical nervous sys- Fig. 130. — Larva of Asterina gibbosa, tem of the larva, and not at viewed as an opaque object from the left ,1 r ii i 1 r 4.1 side. (After LUDWIG.) all from the ectoderm of the ^^^ p^^^^^i ,„i,^_ praeoral lobe. We have thus seen how within the limits of a single group (viz. the Echinoderms) the prasoral lobe can become completely emancipated from the central nervous system ; and we have further recognised the fact that whether the cavity of the prasoral lobe is a derivative of the primary or secondary body-cavity, whether it contains loose mesen- chyme or is lined by an endothelium, the morphological value of the prasoral lobe itself remains the same. THE PR^ORAL LOBE OF THE PROTOCHORDATES. It is probable that the misunderstandings and disagree- ments which are of such frequent occurrence among mor- phologists with regard to the comparison of the types of central nervous system presented respectively by the. 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 work.. Willey, Arthur, 1867-. New York, London, Macmillan

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