The embryology of the vertebrates resembles that of the cephalopod much more than that of the echinoderm. The basic structuring of circulatory, respiratory, renal, and skeletal system in Nautilus shows marked similarities with those in the vertebrates. By contrast, echinoderms lack any distinctive basic adult vertebrate features. The transformation from a nautiloid to a vertebrate would violate no known evolutionary process, and would explain the enigmatic vertebrate pineal eye. It is proposed that the vertebrate originated from an orthoconic nautiloid cephalopod.

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