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Updated: May 3, 2025
Professor Haeckel proposes to break up the vertebrate sub-kingdom, first, into the two provinces of Leptocardia and Pachycardia; Amphioxus being in the former, and all other vertebrates in the latter division.
I can note only a few points which seem to me to be open to discussion. The Monorhina, having been developed out of the Leptocardia, gave rise, according to Professor Haeckel, to a shark-like form, which was the common stock of all the Amphirhina. From this "Protamphirhine" were developed, in divergent lines, the true Sharks, Rays, and Chimaerae; the Ganoids, and the Dipneusta.
Another considerable innovation is the proposition to divide the class Pisces into the four groups of Leptocardia, Cyclostomata, Pisces, and Dipneusta. And there is much to be said in favour of the same promotion of the Cyclostomata, or Lampreys and Hags.
Oldest vertebrates without jaws or pairs of limbs, single nose. : 13. Acrania II. More recent. : 13. Leptocardia. Amphioxus. Oldest vertebrates without jaws or pairs of limbs, single nose. : 14. Petromyzonta larvae. Oldest vertebrates without jaws or pairs of limbs, single nose. : 15. Cyclostoma II. More recent. : 15. Marsipobranchia. Petromyzonta. Silurian. : 16. Selachii. Primitive fishes.
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