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very perfectly preserved fossil specimens of Cephalaspis from the Devonian rocks of Spitzbergen. Serial sections of these specimens have been studied by Professor Patten, who states that the radiating bony channels for the cranial nerves and many other architectural features of the anatomy of the head conform to the general plan seen in the head of the fossil eurypterids and other arthropods. Patten therefore argues that this new material has proved his theory that the vertebrates have been derived from the arthropod stock.

Whether the vertebrates came from very early arthropods or whether they were derived from unknown cigar-shaped forms that preceded both the ostracoderms and the existing lancelets (Amphioxus), it is at least certain that the earliest known ostracoderms already foreshadowed the higher vertebrates, including man, in the ground plan of their organization. Already they had the main chordate characters that are displayed in the human embryonic and foetal stages but that are masked in the adult human stage, namely, a notochord or elastic axis, above which is the central nervous system and below which is the primitive gut and heart.

Like all primitive chordates the ostracoderms swam head on, by throwing the long body into waves proceeding from in front backward. This undulating motion is produced by the rhythmic contractions of a series of zigzagging muscle plates ranged along each side of the body from behind the head to the base of the tail. Each zigzag is separated from the next by a partition of connective tissue which runs inward toward the notochord.

The ultimate unit of locomotion is not the zigzag muscle segment but the short red muscle fibre. Thousands of these little fibres are placed along the zigzag path of the muscle segment, each fibre being attached at its front and rear ends to the connective tissue partitions between the segments.