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).
The Phylum Appendiculata similarly branches into sub-phyla, viz.
Certain additional small groups should probably be recognized as independent lines of descent or phyla, but their relationships are obscure - they are the Mesozoa, the Polyzoa, the Acanthocephala and the Gastrotricha.
- The characters of the subphylum and those of the associated sub-phyla Chaetopoda and Rotifera have been given above, as well as the general characters of the phylum Appendiculata which comprises these great sub-phyla.
So long, however, as our knowledge of these phyla is confined, as at present, to specialized forms, the nature of the relationship between them must remain to some extent hypothetical.