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METAZOA		Metazoa Home

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Metazoa: the Animals

LIFE
|--Eubacteria
`--+--Archaea
   `--Eukarya
      |--Chlorobionta
      `--+--Fungi
         `--METAZOA  
            |--Choanoflagellata  
            `--Porifera 
               |--Radiata
               `--+--Cnidaria 
                  `--Bilateria
                     |--Deuterostomia
                     `--Protostomia
                        |--Ecdysozoa
                        `--Lophotrochozoa

Quick Links to Major Animal Groups

Arthropods: insects, crustaceans, scorpions -- the most successful metazoans.

Brachiopoda: they look like mollusks, but they're not.

Bryozoa: advanced, encrusting reef-builders. 

Chordata: mostly, the Vertebrates. 

Cnidaria: Radiate animals -- jellyfish and corals.

Echinoderms: our cousins, the sea urchins, sand dollars, starfishes and so on. 

Mollusca: clams, oysters, and the like. 

Porifera: sponges. 

"Procoelomates": sometimes bizarre worms near the root of the Bilateria. 

Tardigrada: "water bears." 

Evolution of the Metazoa

The Animal Kingdom (Metazoa) is usually considered to include multi-cellular, heterotrophic eukaryotes in which (unlike Plants) the cells are without cell walls. Everything from sponges and jellyfish to insects and vertebrates is belongs in "Metazoa", and considered to have evolved from a single unicellular choanoflagellate ancestor, sometime during the Ediacaran period.  

From a simple colonial choanoflagellate, animals developed through increasing grades of specialization and complexity: first sponges, then coelenterates, and finally bilateral animals (possessing a head and distinct right and left sides). A recent interpretation of this monophyletic animal kingdom theory is the phylogenetic scheme of Wainright et al. 1993 shows choanoflagellates contained within the monophyletic assemblage Metazoa (= "animals"), and Fungi as the closest sister group to Metazoa.  We thus conceive of Metazoa as the sister group of the Fungi, and use the term phylogenetically to mean toads > toadstools.

The problem here is that, although choanoflagellates seem clearly related to sponges, it is not clear how closely related sponges are to the rest of the Metazoa. It is also difficult to see how such a locally organized organism as a sponge (essentially nothing but a glorified colonial protozoan) can develop into an organism with a specific body structure and internal organs. The most widely held theory seems to be that a colonial choanoflagellate evolved into a hollow spherical ball of cells, the blastula, which constitutes the earliest embryonic stage of development, and even occurs in sponges. The 'blastula model' of metazoan evolution goes back to the famous 19th century German Darwinist Ernst Haekel.

However, it is not certain that such a blastaea animal ever even existed. The theory that "ontogeny recapitulates phylogeny," championed by Haekel (according to which the growing embryo passes through all its past evolutionary stages - e.g. the early human fetus possesses gill slits and a tail), was enormously popular for some time. 

All we can say for certain is that, some time during the Late Proterozoic era, an unknown protozoan (or protistan) organism developed into a tiny colonial form, which eventually became the common ancestor of the Metazoa.  The actual nature of this organism is not known, as it was soft-bodied and left no trace. It used to be thought that sponges evolved from a different single celled organism to higher animals (in which case the Metazoa are a polyphyletic taxon), but recent molecular phylogenetic evidence indicates this is not the case.  

Phylogenetic Organization of Metazoa

The base of the animal tree is moderately non-controversial.  Animals are closely related to a particular group of protists, the choanoflagellates.  The first metazoans were probably sponges, Porifera. Sponges are multicellular animals with specialized cell types, but no specialized tissues.  The different cell types are intermingled, and one part of a sponge looks rather like another.  The Porifera are probably paraphyletic.  That is, all other animals are probably descended from something we would probably think of as a "sponge."  In that sense, all animals may be members of the Porifera.  However, we will use the term only as it applies to sponge-like animals.  More generally, animals without strongly differentiated tissues are sometimes referred to as Parazoa, since there are probably animals other than sponges with this grade of organization, e.g., the Archaeocyatha and the Ediacaran fauna.  

All animals with distinct tissue types are referred to as the Eumetazoa.  Phylogenetically, we might define the Eumetazoa as the crown group uniting jellyfish and silverfish.  The jellyfish lineage includes the Cnidaria (jellyfish, corals, etc.) and some related types.  These animals, the Radiata, seem to have derived from a single common ancestor who was not in our direct line of descent.  The Radiata all have differentiated tissues, but normally nothing much more complicated than "inside" (endoderm) and "outside" (ectoderm), with some level of front-to-back specialization.  From an embryological standpoint, they lack mesoderm, a tissue type which characterizes all more derived animals.  Most are radially symmetrical.  Their body plan is based on a cylinder open at one end, with the open end serving both for ingesting food and eliminating waste.  

All other animals are Bilateria.  Bilaterians undergo a more complex gastrulation and possess mesoderm.  Thus, they begin with three embryonic cell types.  Furthermore, they are bilaterally symmetrical.  In addition to a more complex inside-outside pattern of tissues and the ancestral front-to-back organization, Bilateria have a separate top and bottom, with symmetrical left and right sides.  In many worm-like forms, this top-and-bottom asymmetry is not well developed on the outside.  However, the internal organization generally involves distinct dorso-ventral organization, with, for example, muscles and circulatory structures located dorsally, and the gut and a major nerve chord located ventrally. Most (but not all) also have a separate mouth and anus, so that the flow of digestion is one way.   

At this point, things become much less clear.  There are three groups of Bilateria which show some internal cohesion.  These are the Deuterostomia (including echinoderms and chordates), the Lophotrochozoa (including annelid worms and mollusks), and the Ecdysozoa (including arthropods).  Each of these groups includes some of the "minor phyla" of animals.  However, a large number of mostly worm-like groups are left out of this scheme; and their positions are sometimes too vague even to guess at.  

Of the three main groups, the deuterostomes probably branched off earliest, but even this has been disputed.  The term "protostome" is used a good deal, particularly in the older literature, to refer to the lophotrochozoans, ecdysozoans, and everyone else who exhibits a particular pattern of early embryonic development.  For our purposes, we will treat the deuterostomes as the earliest-branching clade and use the term "protostome" to refer to the crown group of bugs + slugs (Ecdysozoa + Lophotrochozoa).  Other than the most well-established members of the two protostome groups, all other animal phyla will be treated as Bilateria incertae sedis.  

With that said, our tree looks like this:

o : crown group
^ : stem group
¶ : paraphyletic basal radiation
@ : apomorphy-based clade
* : similarity based classical taxon
? : basis not yet established

^ METAZOA (= toads > toadstools)
|--? Choanoflagellata
`--¶ Porifera (paraphyletic)
   |--^ RADIATA 
   `--o EUMETAZOA (= jellyfish + silverfish)
      |--* Cnidaria
      `--o BILATERIA (including ?some/all "Acoelomates") (= starfish + silverfish)
         |--^ DEUTEROSTOMIA (= starfish > silverfish)
         |  |--@ Chordata (notochord present, or ^: movie stars > sea stars)
         |  `--o Echinodermata (Disaster + Pisaster) 
         `--+--? (minor phyla)
            `--o PROTOSTOMIA (= bugs + slugs)
               |--^ ECDYSOZOA (bugs > slugs)
               |  |--* Bryozoa ?
               |  `--? Arthropoda
               |     |--* Crustacea 
               |     `--* Insecta
               `--^ LOPHOTROCHOZOA (slugs > bugs)
                  |--?
                  `--¶ "Procoelomates" (paraphyletic basal lophotrochozoans)
                     |--* Annelida
                     `--+--* Mollusca
                        `--* Brachiopoda

Most of the "Procoelomates" are probably below the deuterostome divergence, perhaps even closer to mollusks than to annelids.

ATW041229.  Slightly revised ATW070725. Text public domain.  No rights reserved.

Some Linnean Lists

We often disparage Linnean lists.  However, The phylogeny of this region is fairly unstable and includes a large number of unknown connections.  Furthermore, most of the invertebrate literature still uses a Linnean scheme for higher-level classifications.  Accordingly, it is useful to include two of these schemes as a point of reference.  In the Linnean system the Animal Kingdom has traditionally been classified into about three dozen phyla, which have been grouped into larger categories: 

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.

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 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.

Modified from Margulis and Schwartz, 1982

Here's another, more recent scheme:

     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 

classification according to Cavalier-Smith, 1998