SINCE zoologists have recognized the simple nature of the Cyclostome fishes there have been many contributions to our knowledge of their structure and many discussions as to their nature and their true zoological position. The increase in our knowledge of the anatomy of amphioxus and the clearing up of its development by the investigations of Kowalevsky, Hatschek, and Willey, have increased the amount of interest in these discussions and have added to the subject an entirely new phase in that

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... e amphioxus, instead of being longer considered a zoological curiosity, a degenerated or aberrant form, has become the center of an intense and searching discussion of the origin and relationships of the Vertebrata; and amphioxus has thus come into its own by being recognized as an ancestral form in the genealogy of the vertebrate stock and the oldest living relative and representative of this group of animals. We can now see clearly enough that the Marsipobranchia and the Acrania both stand in the relation of ancestors to the vertebrates above them, and there is no longer any doubt, while recognizing to the full the many unsolved problems in connection with its *structure and development, that amphioxus belongs to the group of forms, the Prospondylia, predecessors of the Archicrania, from which the Cyclostomes are directly descended. It must 75 This content downloaded from 063.155.