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Updated: August 3, 2026
The Pachycardia are then divided into Monorhina, which contains the Cyclostome fishes, distinguished by their single nasal aperture; and Amphirhina, comprising the other Vertebrata, which have two nasal apertures.
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.
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