Sper,nelo phyla are characterized by an extreme reduction of the gametophyte generation.
The whole " system " or scheme of classification was termed a genealogical tree (Stammbaum); the main branches were termed " phyla," their branchings " sub-phyla "; the great branches of the sub-phyla were termed " cladi," and the " cladi " divided into " classes," these into sub-classes, these into legions, legions into orders, orders into sub-orders, suborders into tribes, tribes into families, families into genera, genera into species.
The Metazoa form two main branches; one, Parazoa, is but a small unproductive stock comprising only the Phylum Porifera or Sponges; the other, the great stem of the animal series Enterozoa, gives rise to a large number of diverging Phyla which it is necessary to assign to two levels or grades - a lower, Enterocoela (often called Coelentera), and a higher, Coelomocoela (often called Coelomata).
Diagram to show the division of the great branch Enterozoa into two grades and the Phyla given off therefrom.
The Phylum Vertebrata in the above scheme branches into the sub-phyla Hemichorda, Urochorda, Cephalochorda and Craniata.
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