This figure illustrates the major branch points in the phylogeny (evolutionary history) of animal diversity. Study this figure; you will see it throughout the tutorials on animals. Each of these bifurcations (branch points) is a discrete dichotomous (division of an ancestral line into two equal diverging branches) point. For example, an organism is either a parazoan or a eumetazoan; eumetazoans are either radiata or bilateria (based upon their symmetry; we will discuss this later). Each bifurcation marks a character state that is thought to clearly separate the lineages at that point. The higher you go in the tree, the more derived (or more recently evolved) is each particular character.
The first dichotomous branching point of the phylogenetic tree of Kingdom Animalia separates organisms that do not have true tissues from those with true tissues. A tissue is an aggregation of cells that performs a function. Parazoans lack true tissues, whereas eumetazoans have true tissues. There has been some debate about whether parazoans, the sponges, should be considered animals; however, molecular data clearly place them at the base of the animal tree. Eumetazoans have distinct collections of cells that are arranged for specific purposes. Thus, the presence of true tissues is the first bifurcation in the animal phylogeny.
Sometimes it can be easier to see similarities between groups at the larval stage. For example, sponge larva are free-swimming with flagellated cells. As adults, they settle in one place and are no longer mobile; they do retain flagella on some of their cells (we will discuss the structure of sponges in more detail later in this tutorial).
The second dichotomous branch point in the Animal phylogenetic tree separates the eumetazoans with radial symmetry (the radiata) from those with bilateral symmetry (the bilateria). In organisms that possess radial symmetry, multiple mirror image planes can be drawn across the organism’s center (think of slicing a pizza; you can slice at any point to divide the pizza into two equal halves). In organisms that possess bilateral symmetry, only one mirror plane can be drawn along a single axis. For example, humans have only one mirror plane, running from head to toe down the center of the body.
Another characteristic of the radiata is that they are diploblastic. Diploblastic organisms only have two embryonic tissue layers: endoderm and ectoderm. The endoderm gives rise to the lining of the digestive tract, and in higher animals, the liver and lungs. The ectoderm gives rise to the animal's outer covering and, in some phyla, the central nervous system. The bilateria are triploblastic and begin life with three distinct tissue layers: endoderm, mesoderm and ectoderm. The mesoderm gives rise to the muscles and most of the internal organs, including the heart and kidneys. We will discuss these tissue types more in the following tutorials. Note that these are embryonic distinctions; this does not mean that these organisms will end up with only two or three tissue types. Some animals that exhibit radial symmetry as adults (for example, the sea stars) have been placed in the bilateria. Why might this be the case? Can an adult morphology differ significantly from the larval morphology of the same animal? Additionally, the radiata tend to be sessile as adults, whereas the bilateria are usually mobile.
The last major branch point of the phylogenetic tree of Kingdom Animalia distinguishes animals with a true coelom (body cavity) based on the fundamental aspects of their development. Protostome coelomates include the mollusks, annelids and arthropods, whereas deuterostome coelomates include the echinoderms and chordates.
One of the main differences between the two is the origin of the openings to their digestive tract. In protostomes the mouth develops first, whereas in deuterostomes the anus develops first. "Stoma" is Greek for "mouth," and "protos" and "deuteros" are "first" and "second," respectively. We will discuss more about the differences between protostomes and deuterostomes later. These two groups also have other differences in the development of their embryos.