From Paleos


Metazoa: the Animals

LIFE (=Archaea?) |--Eubacteria `--Eukarya

           |  `--Ctenophora

Quick Links to Major Animal Groups Evolution of the Metazoa Phylogenetic Organization of the Metazoa Some Linnean Lists Links

Quick Links to Major Animal Groups

Archaeocyatha: enigmatic early reef builders.

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

Lodosiga from Barnes (1968)The Animal Kingdom (Metazoa) is usually considered to include multi-cellular, heterotrophic eukaryotes in which (unlike Plants) the cells are without cell walls. This includes basically everything from sponges and jellyfish to insects and vertebrates included under the heading of to be "Metazoa", and considered to have evolved from a single unicellular choanoflagellate ancestor, sometime during the Vendian 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 front and rear). 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.

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 poorly organized organism as a sponge (essentially nothing but a glorified colonial protozoan) can develop into organisms with a proper 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 all the way back to the famous 19th century German Darwinist Ernst Haskell.

Blastea hypothesis from Barnes (1968)However, it is not certain that such a blastaea animal ever even existed. The ontogeny recapitulates phylogeny theory 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, and incorporated into the late 19th century and early twentieth century esoteric and occult speculations of Blavatsky and Rudolph Steiner, and more recently, the "Up from Eden" transpersonal psychology of Ken Wilber, has been shown to have very little scientific basis, and in fact it is known that Haekel faked a number of his images, chopping up the embryos so they look more like how they should in his theory.

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 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 type of protist, 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. They all have tissues, but normally nothing much more complicated than "inside" (endoderm) and "outside" (ectoderm) tissues, with some level of front-to-back specialization. From an embryological standpoint, they do not undergo gastrulation, and they consequently lack mesoderm, a tissue type which characterizes all more derived animals. Most are radially symmetrical. That is, 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 gastrulation and possess mesoderm. Thus, they begin with three embryonic cell types. Furthermore, the are bilaterally symmetrical. In addition to a more complex inside-outside pattern of tissues and the ancestral front-to-back organization, they have a separate top and bottom. 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 Deuterostoma (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:

METAZOA |--Archaeocyatha `--+--Porifera

  `--EUMETAZOA (= jellyfish + silverfish)
     |  |--Cnidaria
     |  `--Ctenophora
     `--BILATERIA (including "Procoelomates")
        |  |--Echinodermata
        |  `--Chordata
        `--PROTOSTOMA? (= bugs + slugs)
           |  |--Annelida
           |  `--+--Mollusca
           |     `--Brachiopoda
              |--Bryozoa ?

Most of the "Procoelomates" are probably below the deuterostome divergence, and some may be closer to mollusks than to arthropods. However, their relationships are so poorly known that we have "demoted" them to generalized bilaterians for the present.

ATW041229. 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 PROTOSTOMATA - 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 DEUTEROSTOMATA - 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.

external link 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 pdf document Cavalier-Smith, 1998


The Invertebrate Animals - John Kimbell - excellent overview

Checklist of the Invertebrates, Robert B. Hole, Jr.

Invertebrata collection of the Siberian Zoological Museum

Introduction to the Metazoa</a></strong>

Animal Diversity Web University of Michigan Museum of Zoology's Animal Diversity Web

Metazoan Diversity Homepage by Graham Davison - see Prominent Metazoan Evolutionary Schemes and Components - has diagrams showing the possible evolution and relationship of Metazoa (higher animals) from ancestral choanoflagellate forms - a number of figures on that page have been reproduced here

Systema Naturae 2000 / Classification - lists the various animal phyla, and when each division was first formally named and by who; an invaluable reference.

Illinois State Academy of Science The Kingdoms Project The Animal Kingdom at the ISAS

Evaluating Multiple Hypotheses for the Origin of Bilateral Animals by Allen G. Collins. (abstract, the complete essay with diagrams is on a poster)

Chapter 29 - the Animal Kingdom - overview of the main phyla, in notational form

Chapter 32: Introduction to Animal Evolution - explains the characteristics in the divergence of the main metazoan divisions

Suggested Phylogenetic Classification of Invertebrate Animals

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