Palaeos Palaeos Metazoa
Metazoa Classification

Metazoa: Classification

Some Linnaean Lists

The classification of the animal kingdom according to major ctegories goes back to Aristotle and, more recently, Linnaeus, but it was on in the 19th century that scientists like Lamarck (who developed invertebrate zoology), Cuvier, and Haeckel developed the idea of major groups or phyla. Similarily, the relation of the colemate phyla was determined by the most apparent similarities. For Lamarck and Haeckel the idea of evolution in the conbtext of a linear chain of being, was central. Haeckel's famous "Ascent of Man" tree (and numerous and somewhat less anthropocentric 20th century versions thereof) presented the idea of progressive evolutionary stages, with protozoa at the bottom, then sponges jellyfish, and flatworms, then higher invertebrates, and finally at the top something like mammals and butterflies, reflecting anthropomorphic preferences, e.g. cuddly (mammals) or aestheric appeal (butterflies), with man ubiquitously placed at the summit (either alone or with other higher forms of vertebrate and invertenrate life). More sophisticated five and six kingdom models of life (including a rational arrangement of animal phyla) were presented by Margulis and Schwartz (1982) and Cavalier-Smith (1998) respectively, and these represent the most sophisticated developmen to date of the Linnaean paradigm.

From the 1990s onwards, molecular phylogeny and cladistics revolutionised our understanding and classification of the animal kingdom, and replaced the rank-based system with a branching tree-based model. However, The details of the branching pattern (the phylogenetic hypotheses) differ according to different researchers and analyses, and there are still a large number of unknown connections. Furthermore, most of the invertebrate zoology literature still use the traditional Linnaean scheme for higher-level classifications.  Accordingly, it is useful to include a few of these schemes as a point of reference.  In the the Animal Kingdom has traditionally been classified into about three dozen phyla, which have been grouped into larger categories: 

The following is adapted from Margulis and Schwartz, 1982 and still represents very much the standard biology textposition view

Kingdom ANIMALIA - Develop from a blastula, cellular to organ-systems grade, food ingesting without chloroplasts, subdivided on grade of organization, symmetry, and coelomic development.

Subkingdom PARAZOA - Cellular (multi-cellular) grade, no tissues, organs, digestive tract or mouth.

Phylum Placozoa

Phylum Porifera - porous with one to many internal cavities lined with choanocytes; (the sponges).

Subkingdom EUMETAZOA - Tissue to organ-system grade, with mouth and digestive tract.

Branch RADIATA - Radial to modified radial symmetry, tissue grade organization with incipient organs, diploblastic, mesenchyme of ectodermal origin, digestive cavity the sole body cavity, no anus.

Phylum Cnidaria - Symmetry radial, biradial, or radio-bilateral, mouth usually encircled by tentacles armed with nematocysts; (the coelenterates, jellyfish).

Phylum Ctenophora - Symmetry biradial, eight radial rows of ciliated swimming plates, tentacles when present not encircling mouth, no nematocysts.

Branch BILATERIA - Primary bilateral symmetry, secondarily modified to pentameral or radial, organ-system grade of organization, most triploblastic with well-developed mesoderm of endodermal origin, most with body cavity other than the digestive cavity, anus typically present.

[note: the following traditonal division of Bilateria into Acoelomata, Psuedocoelomata, and Coelomata is now known to be invalid, see discusison under Ecdysozoa)

Grade ACOELOMATA - No coelom, region between digestive tract and body wall filled with mesenchyme or mesoderm, if segmented youngest segments nearest head.

Phylum Mesozoa

Phylum Platyhelminthes

Phylum Nemertina

Phylum Gnathostomulida

Grade PSEUDOCOELOMATA - Body cavity a pseudocoel (remnant of blastocoel, not lined with mesoderm on both sides), triploblastic.

Phylum Gastrotricha

Phylum Rotifera

Phylum Kinorhyncha

Phylum Acanthocephala

Phylum Entoprocta

Phylum Nematoda

Phylum Nematomorpha

Grade COELOMATA - With a true coelom and well-developed mesoderm.

Series Protostomia - Blastopore becomes mouth, typically schizocoelous with spiral cleavage.

Phylum Bryozoa (Ectoprocta) - Colonial lophophorate, oligomerous.

Phylum Phoronida - Solitary lophophorate with worm-like (vermiform) body, oligomerous.

Phylum Brachiopoda - Solitary lophophorate with bivalve shell, oligomerous, enterocoelous.

Phylum Mollusca - Pseudometamerous, reduced coelom, visceral mass covered by a body fold, the mantle, which secretes a calcareous shell of one or more pieces.

Phylum Priapulida - Marine worms, some consider pseudocoelomate; no fossil record.

Phylum Sipuncula - Marine worms, amerous; no fossil record.

Phylum Echiura - Another amerous worm.

Phylum Annelida - Metamerous, segmented, vermiform, without jointed appendages.

Phylum Tardigrada - small (<2mm) worm-like, meiofaunal, no fossil record.

Phylum Pentostomata - "Tongue worms", parasitic, no fossil record. [note: now known to be a specialized side-branch of arthropods)

Phylum Onychophora - Metamerous, segmented, uniramous unsegmented appendages, waxy cuticle.

Phylum Arthropoda - Metamerous, segmented; uniramous or biramous jointed (segmented) appendages.

Series DEUTEROSTOMIA - Blastopore becomes anus, typically enterocoelous with radial cleavage, and oligomerous.

Phylum Pogonophora - "Beard worms", sessile deep-sea worms that build chitinous tubes, some large forms inhabit hydrothermal vents.

Phylum Echinodermata - With secondary, pentamerous radial symmetry; water vascular system; calcareous endoskeleton of mesodermal origin.

Phylum Chaetognatha - "Arrow worms" and conodonts; without gill slits or endoskeleton; "teeth" of calcium phosphate (apatite).

Phylum Hemichordata - With gill slits and nerve chord; no notochord.

Phylum Chordata - "Vertebrates"; gill slits, nerve chord and notochord; endoskeleton of mesodermal origin.

Cavalier-Smith (1998) is predictably idiosyncratic yet insightful. His including Porifera under Radiata did not catch on and feels less useful than Margulis and Schwartz's distinction of Parazoa and Eumetazoa. On the plus side the old Acoelomata, Psuedocoelomata, and Coelomata are rejected in favour of more recent taxa such as Lophozoa (= Lophotrochozoa = Spiralia) and the important molecular phylogenetic revealed Ecdysozoa:

     Kingdom Animalia 
        Subkingdom Radiata 
           Infrakingdom Spongiaria 
              Phylum Porifera 
           Infrakingdom Coelenterata 
              Phylum Cnidaria 
           Infrakingdom Placozoa 
       Subkingdom Myxozoa 
       Subkingdom Bilateria 
         Branch Protosomia 
          Infrakingdom Lophozoa 
             Phylum Bryozoa 
             Phylum Kamptozoa 
             Phylum Mollusca 
             Phylum Brachiopoda 
             Phylum Sipuncula 
             Phylum Annelida 
             Phylum Nemertina 
          Infrakingdom Chaetognathi 
             Phylum Chaetognatha 
          Infrakingdom Ecdysozoa 
             Phylum Arthropoda 
             Phylum Lobopoda 
             Phylum Nemathelminthes 
          Infrakingdom Platyzoa 
             Phylum Acanthognatha 
             Phylum Platyhelminthes 
         Branch Deuterostomia 
          Infrakingdom Coelomopora 
             Phylum Hemichordata 
             Phylum Echinodermata 
          Infrakingdom Chordonia 
             Phylum Urochorda 
             Phylum Chordata 
       Subkingdom Mesozoa 

A preferable model would combine the above two classifications, along with taxa based on the research of recent workers such as Dunn et al. 2008 and Edgecombe et al 2011 MAK120421

page uploaded 7 April 2002; last modified ATW070725, checked ATW070725; last modified MAK120421