Click HERE for your print-friendly copy of the notes. Don't print the big font pages!

The Wonderful World of Animalia

What is an animal?

Animalia: Emerging Views of Phylogeny

(Phylogenies change as new data become available. The most recent phylogeny believed to be correct for Animalia can often be found at the Tree of Life Web Project.)

As we begin our Tiptoe through the Tulips...erm...Taxa, keep track of new characteristics as they appear in the more derived taxa.

Where did Animals come from?

Animals are believed to share a most recent common ancestor with the primitive protists known as choanoflagellates.

Sponges actually have choanocytes (collar cells) that are extremely similar to these choanoflagellates, and since they are the first type of cell to develop in sponges, there is strong evidence that other types of animal cells may be been derived from choanoflagellate cells.

The mechanism of transition from unicellular to multicellular organisms is unknown, but several gene families have been implicated in the origin of multicellularity, which may have evolved several times, independently.

Among the most important is the cadherin gene family.
Cadherins ("calcium-dependent adhesion" proteins; they require calcium (Ca2+) to function) are trans-membrane proteins involved in both cell adhesion and intercellular signaling.

Extant colonial choanoflagellates have cadherin genes, and their ancestors may have developed into a protoanimal resembling a gastrula:

...but which one might call a gastrea to distinguish it from an embryo, since this early animal was a mature, reproductive organism.

A gastrea-like creature was most likely the ancestor of all Eumetazoans.
It shared these characters with modern animals:

One lineage of gastreas may have evolved into flattened, wormlike creatures only slightly more complex than a gastrea.
These may have looked something like members of the extant taxon "Acoela":

These extant marine flatworms are among simplest of all eumetazoans, and are considered basal to them.


The literal translation of the German bauplan is "a structural plan or design." But when the word is applied to animal groups, it is more than that:

As we tour the Metazoans, note and anticipate the increasing in complexity of animal body plans.

Meet the Metazoa

Metazoans can be broadly grouped into three major categories based on body symmetry:

Porifera: The Sponges (<--required video link)

The name is from the Greek por - "small hole" and fer - "to bear".

Porifera is a form taxon, a grouping based on superficial similarity.
The evolutionary relationships among the sponges are still being determined.
They share a common cellular division of labor that may have evolved several times.

General Anatomy of a Sponge.

There are no true tissues, hence no true plane of symmetry, even in forms that superficially appear radial or bilateral.

Sponges are composed of four types of cooperative cells

Various species of sponges may exhibit three levels of "folding".
From simplest to most complex:

Three Main Groups of Sponges

Eumetazoa: The True Animals

All animals other than sponges have...

Animals may be characterized by the nature of the internal body cavity (located between the ectoderm and endoderm.

  • acoelomate (ay-SEE-low-mate)
    no internal body cavity

  • pseudocoelomate (SOO-doh-see-low-mate)
    internal body cavity lined with mesoderm only on parietal surface, and is known as a pseudocoelom or pseudocoel

  • coelomate (SEE-low-mate)
    internal body cavity lined with mesoderm on both visceral and parietal surfaces, and is known as a coelom (SEE-lohm)

    If a true coelom is present, the animal can be characterized by the course of its embryonic development:

    And another 1000 words...

    Here are some helpful animations.


    More derived animal lineages exhibit an important anatomical innovation, segmentation (also known as metamerism, with each segment called a metamere or somite).

    Muscles, organs and other anatomical structures are duplicated in each segment, with segments arranged in serial fashion.

    Tagmatization is the developmental fusion of groups of body segments (metameres) into functionally distinct body regions, or tagmata (singular = tagmatum). The classic example is the division of the arthropod body into the head, thorax and abdomen, each of which is developed via the fusion of embryonic metameres.

    As evolution proceeded, some animals that had segmented ancestors secondarily lost their segmentation.
    Can you think of any part of your body that's a remnant of your segmented ancestral heritage?


    Bilaterally symmetrical animals have an advantageous anatomical feature: cephalization. This is the presence of a cephalon (Greek for "head" at the front of the body, where the sense organs are concentrated.

    Body plane terminology:

    Now go examine yourself in the mirror and be able to identify all those planes and points.