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  Section: General Zoology » The Diversity of Animal Life
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Major Subdivisions of the Animal Kingdom

Classification and Phylogeny of Animals
Linnaeus and the Development of Classification 
Taxonomic Characters and Phylogenetic Reconstruction 
Theories of Taxonomy 
Major Divisions of Life 
Major Subdivisions of the Animal Kingdom 

Major Subdivision of the Animal Kingdom
The phylum is the largest formal taxonomic category in the Linnaean classification of the animal kingdom. Animal phyla are often grouped together to produce additional, informal taxa intermediate between the phylum and the animal kingdom. These taxa are based on embryological and anatomical characters that reveal the phylogenetic affinities of different animal phyla. Zoologists in the past have recognized subkingdom Protozoa, which contains the primarily unicellular phyla, and the subkingdom Metazoa, which contains the multicellular phyla. As noted above, however, Protozoa is not a valid taxonomic group and does not belong within the animal kingdom, which is synonymous with Metazoa. The traditional higher-level groupings of true animal phyla are as follows: Branch A (Mesozoa): phylum Mesozoa, the mesozoa Branch B (Parazoa): phylum Porifera, the sponges, and phylum Placozoa Branch C (Eumetazoa): all other phyla Grade I (Radiata): phyla Cnidaria, Ctenophora Grade II (Bilateria): all other phyla Division A (Protostomia): characteristics in Figure 10-13 Acoelomates: phyla Platyhelminthes, Gnathostomulida, Nemertea Pseudocoelomates: phyla Rotifera, Gastrotricha, Kinorhyncha, Nematoda, Nematomorpha, Acanthocephala, Entoprocta, Priapulida, Loricifera Eucoelomates: phyla Mollusca, Annelida, Arthropoda, Echiurida, Sipunculida, Tardigrada, Pentastomida, Onychophora, Pogonophora Division B (Deuterostomia): characteristics in Figure 10-13 phyla Phoronida, Ectoprocta, Chaetognatha, Brachiopoda, Echinodermata, Hemichordata, Chordata

As in the outline, bilateral animals are customarily divided into protostomes and deuterostomes on the basis of their embryological development (Figure 10-13). However, some of the phyla are difficult to place into one of these two categories because they possess some characteristics of each group (See: Lesser Protostomes and Lophophorate Animals ).
Basis for the distinction between divisions of bilateral animals
Figure 10-13
Basis for the distinction between divisions of bilateral animals.

Recent molecular phylogenetic studies have challenged traditional classification of the Bilateria. Molecular phylogenetic results place four phyla classified above as deuterostomes (Brachiopoda, Chaetognatha, Ectoprocta, and Phoronida) in the Protostomia. Furthermore, the traditional major groupings of protostome phyla (acoelomates, pseudocoelomates, and eucoelomates) appear not to be monophyletic. Instead, protostomes are divided into two major monophyletic groups called the Lophotrochozoa and Ecdysozoa. Reclassification of the Bilateria is as follows:Grade II: Bilateria Division A (Protostomia): Lophotrochozoa: phyla Platyhelminthes, Nemertea, Rotifera, Gastrotricha, Acanthocephala, Mollusca, Annelida, Echiurida, Sipunculida, Pogonophora, Phoronida, Ectoprocta, Chaetognatha, Brachiopoda Ecdysozoa: phyla Kinorhyncha, Nematoda, Nematomorpha, Priapulida, Arthropoda, Tardigrada, Onychophora Division B (Deuterostomia): phyla Chordata, Hemichordata, Echinodermata Further study is needed to confirm these new groupings, and to add to the classification four phyla (Entoprocta, Gnathostomulida, Loricifera, and Pentastomida) whose relationships have not been determined. We organize our survey of animal diversity using the traditional classification, but discuss implications of this new one.


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