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The Cambrian Explosion

For its first four billion years, earth was populated, initially, only by prokaryotic organisms, and later by very simple metazoan life forms. This was the Precambrian Period, and there were no rabbits.

But about 540 million years ago, the diversity of marine animals suddenly expanded dramatically, an event known as the Cambrian Explosion or (less dramatically) the Cambrian Radiation.

Recall that the (relatively) rapid diversification of life forms from a single ancestor is known as adaptive radiation.

The Cambrian Explosion saw the evolution of modern animal phyla--and even more that are now extinct--from simple, ancestral forms.

Molecular clock studies suggest that...

Fossil evidence does not go back that far (only about 550 million years), but that doesn't mean the ancestors didn't exist at an earlier time.

Eumetazoan Diversity

Eumetazoans are defined by gastrulation and the presence of true embryonic germ layers (endoderm, ectoderm, mesoderm) that develop into true tissues.

Eumetazoans can be broadly divided into two main groups, based on body similarity and number of germ layers:

Radially Symmetrical Animals

These diploblastic organisms are among the simplest of modern animals.
In the radially symmetrical animals we first see...

A. Placozoa

These simplest of all animals are of uncertain evolutionary affinity.
They are little more than a jelly-like plate of interdependent cells exhibiting the beginnings of radial symmetry.
There is only one recognized extant species, Trichoplax adhaerens.

B. Cnidaria

The name of the taxon derives from the Greek knide meaning "nettle."

The Cnidarian Bauplan

Major Groups of Extant Cnidarians:

Anthozoa - polyp is the dominant life cycle phase; medusa is absent
Scyphozoa - medusa is the dominant phase; polyp is reduced
Hydrozoa - polyp and medusa phases alternate

C. Ctenophora - Comb Jellies

  • radially symmetrical
  • diploblastic
  • no stinging cells
  • voracious carnivores

    Paired rows of cilia along the body axis beat in waves to propel the animal through the water.

    Their tissues are colorless and translucent, with the same refractive index as water.
    This makes them quite cryptic in their natural habitat, and well concealed from predators.
    But when the light catches them just right...

    Until recently, ctenophores were believed to have a mouth but no anus.
    Newly discovered anal pores have stunned evolutionary biologists.


    Animals exhibiting bilateral symmetry are a vast, diverse assemblage.
    We'll start from (what we believe are) the most primitive forms.

    Click on pic for primary literature link!

    Basal Bilaterians: Flatworms

    Molecular data suggest that some members of a large group of free-living flatworms, the Acoela, are descended from the earlist offshoots of the first ancestral bilaterally symmetrical animal.

    Acoel flatworms exhibit the earliest occurrence of

    Acoels ingest food via their syncytium, which engulfs and creates vacuoles around food particles.

    A short, sometimes eversible, pharynx leads to the syncytium/vacuole.

    Acoels lack circulatory, respiratory or excretory systems.

    Acoels lack nerve ganglia or any form of brainlike structure.

    They are hermaphroditic, but lack gonads (ovaries or testes). Both eggs and sperm are derived from mesemnchymal cells.

    Mysterious and ancient.

    Platyhelminthes: More Complex Flatworms

    Turbellaria - The free-living (non-parasitic) flatworms.
    Trematoda - Flukes (all parasitic)
    Cestoda - Tapeworms (all parasitic)

    The Platyhelminth Bauplan

    Platyhelminths are acoelomate. They have...

  • reproductive system (all hermaphroditic)
  • excretory system

    Platyhelminths do NOT have:

    Turbellaria - The Planarians (free-living flatworms)

    The planarians are the most cephalized flatworms, as they live free in the environment. Their nervous systems and other organisms are the best developed of the flatworms.

  • Chemoreceptors are located on flaplike extensions of the head called auricles, though they have nothing to do with hearing.

  • Photoreceptors are simple ocelli (singular - ocellus).
  • Statocysts (gravity sensing cells) occur near the cerebral ganglion.

    Some marine forms exhibit aposematic (warning) coloration and are extremely toxic.

    If you just can't get enough, visit Flatworms of the World.

    Trematoda - The Flukes

    The flukes are entirely parasitic, and often have complex life cycles.
    Their sensory systems and other organ systems are reduced, as compared to the free-living planarians. (Why?)

    (click on the photo for a required link!)

    Parasitology Lingo

    Most species of parasite are relatively host-specific, but some can inhabit more than one different species of definitive host.

    In many life cycles, more than one intermediate host is required for the full life cycle to be completed.

    Transmission of parasites (or any pathogen, for that matter) may be

    Cestoda - The Tapeworms

    The tapeworms are entirely parasitic, and have the most reduced organ systems of all the flatworms.

  • hermaphroditic Tapeworm anatomy and function