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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:

I. 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.


B. Cnidaria

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

The Cnidarian Bauplan

Major Groups of Extant Cnidarians:

1. Hydrozoa - polyp and medusa phases alternate


2. Scyphozoa - medusa is the dominant phase; polyp is reduced
3. Anthozoa - sea anemones, corals, sea pens, etc. (polyp is dominant stage; medusa is reduced or absent.)



C. Ctenophora (Comb Jellies)

  • radially symmetrical
  • diploblastic
  • mouth, but no anus
  • (no cnidocytes)
  • The "comb jellies" are so named because of paired rows of cilia along the axis of their bodies that beat in waves to propel the animal through the water (slowly, but surely).
  • They are suspension feeders, with a pair of long, sticky tentacles that they drape out into the water to capture microscopic plankton and detritus. Every now and then, the ctenophore wipes its tentacle along its mouth to remove the food particles captured there.
  • 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 to the human eye, they are spectacular in motion.

    Cnidarians and Ctenophores are beautiful!



    II. Bilateria

    The bilaterally symmetrical animals are a vast assemblage of animals that exhibit bilateral symmetry: in longitudinal section, the two halves of the body form mirror images.

    Basal Bilaterians: Flatworms

    Flatworms comprise the (probably polyphyletic) taxon Platyhelminthes (literally "flat" (platy) "worm" (helminth).
    Molecular data suggest that some members of a large group of free-living flatworms, the Acoela, are actually extremely early offshoots of the very first bilaterally symmetrical ancestor.

    A. Acoela
    In these primitive bilaterians, we see the origin of:

  • primordial gut precursor (By definition, a true intestine must be lined with epithelial cells. In these, it is not.
  • the ingestive structure is a syncytium that creates vacuoles around ingested food
  • a short, sometimes eversible, pharynx leads to the syncytium/vacuole
  • no circulatory, respiratory or excretory systems
  • no nerve ganglia or brainlike structure
  • cephalization vague: slightly more nervous tissue at one end
  • the only sense organs are statocysts (gravity detector) and sometimes a light detecting ocellus (plural = ocelli)
  • hermaphroditic, but no gonads: produce both eggs and sperm from mesenchymal cells Mysterious and ancient.


    B. Turbellaria - Free-living (non-parasitic) triploblastic flatworms, a.k.a. "planarians"
    C. Trematoda - Flukes (all species parasitic)
    D. Cestoda - Tapeworms (all species parasitic)

    The Platyhelminth Bauplan

    Flatworms DO have:

  • reproductive system
  • excretory system

    They 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.


    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. Nervous system is vestigial, and digestive system is secondarily lost. Transport of nutrients, oxygen, and waste takes place across the integument.

    The head of the animal is the scolex, from which sprout segments called proglottids. Each early proglottid contains both ovaries and testes (tapeworms are hermaphroditic), and older proglottids contain hundreds of eggs which are shed with the host's feces.


    Let's look at some pretty flatworms.