Tissue Level of Organization


     The most primitive animals are multicellular organisms that lack tissues.  The lineage of animals which resulted in archaeocyathans and modern sponges was the earliest branch from the family tree of mammals whose descendants survive today.  The next evolutionary step for animals was a tissue level of organization.  Tissues are collections of cells and cell products that perform a common function.  Primitive animals with tissues gave rise to a number of lineages, including those which led to modern cnidarians (a group which includes jellyfish, corals, sea pens, and sea fans). 


  The following images are of cnidarian tissues:

(Hydra in the first image; sea anenomes in the next two images)

cnidarian tissue cnidarian tissue
cnidarian tissue

Animals such as jellyfish which achieve only the tissue level of organization also have the primitive condition of radial symmetry: rather than having right and left halves, there is more than one plane of symmetry around a central axis.  Organisms with radial symmetry are equally receptive on all sides but have few specialized structures such as eyes (because then they would need them for all sides of their body).


During the Ediacaran Period, radially symmetrical animals outnumbered bilaterally symmetrical animals. Ever since then, bilaterally symmetrical animals have been predominant (McCall, 2006).

     Most cnidarians are marine and all are carnivorous.  They have tissues but no organs and their digestive cavity has one opening which serves as both their mouth and anus. Jellyfish and sea fans are known since Ediacaran fauna of the Precambrian but do not fossilize well (given their absence of hard parts) and their fossil record is sparse.

There is some evidence that the comb jellies (Ctenophores) evolved separately from other cnidarians and there have been conflicting reports over which is most closely related to bilaterans. Some ctenophores possess striated muscle in their tentacles (Seipel, 2005).


     Corals are more frequently fossilized because they can secrete hard skeletal elements.  Coral reefs (which can be enormous--the Great Barrier Reef is 2,000 km long) are the basis for many marine communities.  Tabulate corals appear in the Ordovician Period and become extinct at the end of the Permian Period (refer to the photos below which have horizontal lines or tabulae running through them).  Although they were an important component of reef communities, they were not the basis of reef communities in the way that modern corals are.  Rugose or horn corals appear in the Ordovician and become extinct at the end of the Permian.  All of the Paleozoic and early Mesozoic corals lived predominantly in shallow water.  Modern corals (scleractinian corals) first appear in the Mid-Triassic and the earliest members were not reefbuilders.  Although reefs have long been the basis for marine communities, they have varied greatly over time—the first were composed by the archaeocyathans, then sponges, then tabulate and rugose corals, then finally modern corals (Prothero, 1998; Stanley, 2001; Donovan, 2000b).

fossil coral fossil coral
fossil coral
fossil coral
fossil coral
The following are images of modern corals.
coral coral
coral coral

C) Organ Level of Organization

     All animals with organs have bilateral symmetry: the only way to divide them into two equal halves is to divide them into right and left halves.  Sense organs tend to be located at one end, the end that is usually propelled forward.  With sense organs concentrated at the front end of early bilaterans, nervous tissue began to accumulate there to process this sensory input, thus forming a brain.



     The word “worm” is used to describe diverse groups of organisms, some of which are less closely related to each other than they are to animals such as ourselves.  The most primitive worms, the flatworms, still show evidence of their ancestry from radially symmetrical organisms.  The mouth is still located in the center of the organisms (instead of at the head) and there is only one opening to the digestive tract (that is, there is no anus).  In the most primitive flatworms, the acoels, the nervous system is little more than a diffuse nerve net, similar to that found in jellyfish.

  Many worms are microscopic, as is this roundworm.


The following images are from planaria, which are very simple worms:

Note the digestive tract which is highly branched to distribute materials throughout the worm (which is useful since planaria lack circulatory systems).

planarian planarian


    All higher animals (including ourselves) are referred to as coelomates because they possess a coelom.  What is a coelom?  A coelom is an internal body cavity in which a number of organs are hung (in humans the coelom contains the heart, lungs, stomach, liver, and the intestines).  In the following cross section of a primitive chordate, note that the gastrointestinal tract (GI tract) and gonads form in a cavity unlike the somites (blocks of muscle) or the notochord.

Our organs are suspended in a cavity and surrounded by space, rather than being fused to surrounding tissues.  You can see the space around the organs in the coelom of a shark, frog, and opossum below.
shark frog


    Flatworms lack a coelom and other worms are referred to as “pseudo-coelomates”.  The most advanced worms are true coleomates as are all groups of higher animals which evolved from worms (such as mollusks, arthropods, and vertebrates).  Worm trails are present in Ediacaran strata.  The worms of the fossil record are very diverse as the images below indicate. 

The worm Paleoscolex piscatorum is known from the Lower Ordovician and is thought to represent a priapulid worm. Similar worms are known from the Burgess Shale and other Cambrian deposits, indicating that these worms were abundant organisms of the ocean floor of the Early Paleozoic. This type of worm was segmented with a covering of plates called sclerites. Many were only several millimeters in size (Morris, 1997).

Worms, including segmented worms are known from the Cambrian; some of them even are preserved as developing embryos.

worms worms
     The unusual animal Wiwaxia (below) was once thought to represent a distinct phylum, now it is recognized as a specialized annelid (Levinton, 1992).

Fossils from the earliest Cambrian include embryos of segmented worms (Bengston, 1997).Worms lack the specialized limbs that are present in most higher animals such as vertebrates and arthropods.  Although the limbs of arthropods and vertebrates seem to be entirely unrelated to each other, there are shared genetic mechanisms in the development of vertebrate and arthropod limbs.  These mechanisms are also shared with the formation of the development of parapodia and antennae in annelid worms and tube feet in echinoderms.  It seems that genetic mechanisms evolved to allow specialization of the lateral body wall of simple Precambrian bilaterans which higher animals elaborated for the formation of specialized limbs.  The parapodia of some worms may be similar to the most primitive form of limbs.  Thus, much of the great diversity of seemingly unrelated animal appendages may be homologous to some degree (Panganiban, 1997; Shubin, 1997).