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Animals I - An Overview of Phylogeny and Diversity

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You should have a working knowledge of the following terms:

  • acoelomate
  • archenteron
  • bilateral symmetry
  • bilateria
  • blastocoel
  • blastomere
  • blastopore
  • blastula
  • cellular differentiation
  • choanoflagellate
  • cleavage
  • coelom
  • coelomate
  • determinate development
  • deuterostome
  • development
  • diploblastic
  • ectoderm
  • endoderm
  • eumetazoa
  • gastrula
  • gastrulation
  • indeterminate development
  • induction
  • larva (pl. larvae)
  • mesoderm
  • morphogenesis
  • parazoa
  • pattern formation
  • phylogeny
  • protostome
  • pseudocoelomate
  • radial symmetry
  • radiata
  • tissue
  • totipotency
  • triploblastic

Introduction and Goals

This tutorial will begin our discussion of Kingdom Animalia. First, we will discuss what makes an organism an animal. Second, we will focus on some major milestones in the evolution of animals. Third, we will conclude with a discussion of animal development (from a single-cell zygote through a specialized embryo). The next four tutorials will focus on animal diversity. By the end of this tutorial you should have an understanding of:

  • The general characteristics of all animals
  • The evolution of animals
  • The major branch points in the phylogeny of animal evolution
  • The basic steps involved in animal development

Figure. 1 (Click image to enlarge)

What are Animals?

We all agree that a cat is an animal, but what is it that makes it an animal? Why is a sponge an animal? Why are sponges and cats placed in the same kingdom? You might think that some corals look more like plants, and that sponges don't look like much of anything; at least, not like anything alive.

Figure. 2 (Click image to enlarge)

There are some basic features that are found in all of the members of the kingdom Animalia. In general, animals are all motile, heterotrophic, and multicellular.

Motility refers to the ability to move and/or affect motion in one's vicinity. For example, adult sponges don't move from place to place, but they can manipulate their cells to bring food to themselves. Also, while some animals are sessile as adults, all animals exhibit movement from place to place sometime in their development. For example, sponges and corals are motile as larvae (the very early stages of embryonic development) and sessile as adults. A myriad of adaptations for motility are observed in animals, and some of these will be addressed in this tutorial. The image on the right depicts the development of a sponge.

Figure. 3(Click image to enlarge)

Animals are ingestive heterotrophs (they ingest nutrients). Unlike plants, who store their food as starch, animals store their food as glycogen.

Animals have muscle tissue and nervous tissue. These tissues can range from being indistinct (as observed in sponges) to being highly complex (as observed in vertebrates). The coordination between muscle tissue and nervous tissue can result in very specialized movements (e.g., food procurement, mate pursuit, predator avoidance).

Figure. (Click image to enlarge)

When Did Animals Arise?

Fossil records support the hypothesis that the first animals appeared approximately 700 million years ago, but some researchers think that animals might have evolved much earlier. Penn State researcher, Blair Hedges, has estimated that animals may have arisen 1.6 billion years ago.

The Origin of Animals

The first animals likely originated from a colonial protist similar to the choanoflagellate shown here. Choanoflagellates have many of the basic features observed in animals.

Figure. 5(Click image to enlarge)

Note that this colony of individual cells is anchored to a substrate, and that its flagellum sweeps food into a cell for ingestion. The cell can then transfer the food to other cells. While this scenario demonstrates little specialization, it is a very primitive form of multicellularity and ingestive heterotrophy.

Animal Evolution: The Big Picture

This figure illustrates the major branch points in the phylogeny (evolutionary history) of animal diversity. Study this figure; you will see it again.

Figure. 1 (Click image to enlarge)

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 (to be discussed). Each bifurcation marks an ancestral character state that is thought to clearly separate the two major lineages. The higher in the tree, the more derived (or more recently evolved) is that particular character.

Parazoa Versus Eumetazoa

The first dichotomous branch 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 aggregate of cells that performs a function. Parazoans lack true tissues, whereas eumetazoans have true tissues. There has been some debate about whether parazoans, namely the sponges, should even be considered animals. Eumetazoans have distinct collections of cells that are arranged for specific purposes. Because tissue organization is the first bifurcation, it is the most basic criteria for assigning animals to phyla.

Figure. 1(Click image to enlarge)

Sometimes it is easiest to see similarities between groups at the larval stage. It is on this basis, the similarity between sponge and eumetazoa development, that sponges are classified as animals.

Figure. 3(Click image to enlarge)

The Eumetazoans: Radial Versus Bilateral Symmetry

The second dichotomous branch point of the phylogenetic tree of Kingdom Animalia separates the eumetazoans with radial symmetry (the radiata) from those with bilateral symmetry (the bilateria). In organisms that possess radial symmetry, multiple mirror planes can be drawn (think of a wheel). 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.

Figure. 1 (Click image to enlarge)

Another pivotal 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. 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 have been placed in the bilateria. Why might this be the case? That is, do you think an adult morphology might differ significantly from the larval morphology of the same animal?

Additionally, the radiata tend to be sessile as adults, whereas the bilateria are motile. We will discuss this distinction later in the tutorials.

Acoelomates Versus Coelomates

The third major bifurcation of the phylogenetic tree of Kingdom Animalia distinguishes animals by whether or not they have a body cavity. Coelomates have a body cavity, whereas acoelomates do not. Note, there is also a distinction made between pseudocoelomates and "true coelomates" (discussed in the next section). A coelom is a fluid-lined space (body cavity) that separates the gut from the outer body wall. However, don't confuse "body cavity" with "gut" because they are not the same thing. For example, our intestines are suspended within our body cavities.

Figure. 1(Click image to enlarge)

In an acoelomate gut, muscle contractions are not buffered by a fluid-filled body cavity; therefore, all forces generated during a contraction are transmitted throughout the animal and affect all internal organs. The coelom serves as a mechanical buffer in coelomates; it helps protect internal organs from shock. Fortunately for us, we can run and jump without bouncing our heart and lungs around too much.

Pseudocoelomates Versus Coelomates

The figure on the right illustrates the body plans of the bilateria. Note the acoelomate, pseudocoelomate and coelomate conditions. Do you see that pseudocoelomates (e.g., nematodes) have a body cavity that is only partially lined by mesoderm-derived tissue? Keep in mind that mesoderm gives rise to muscle (as well as other things). Think about the coelomate versus pseudocoelomate state in respect to muscle distribution and ask yourself if you think a pseudocoelomate can move its body independent of the gut cavity?.

Figure. (Click image to enlarge)

Protostomes Versus Deuterostomes

The last major branch point of the phylogenetic tree of Kingdom Animalia distinguishes coelomates based on the fundamental aspects of their development. Protostome coelomates include the mollusks, annelids and arthropods, whereas deuterostome coelomates include the echinoderms and chordates.

Figure. 1(Click image to enlarge)

One of the main differences between the two is the origin of their gut. 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.

Animal Development

In the kingdom Animalia, organisms are primarily grouped according to their development. Developmental continuity throughout the kingdom is impressive. That is, two animals can look and behave very similarly when in a young developmental stage but be very different as adults.

Development is defined as those processes that are irreversible and occur from zygote formation to death. The word "irreversible" is key and limits the definition of development. Cellular differentiation is defined as how a cell diverges from its early morphology into a more specialized morphology, in an irreversible manner. Morphogenesis is how an organism's "shape" is acquired, and pattern formation describes how cells, tissues, and organs are arranged in an organism.

The development of a diploid organism begins with fertilization. Shortly after the gametes unite to form the single-cell zygote, cell division commences. These early cell divisions are called cleavage. Cell division is not a random process, and various patterns of cleavage are observed. In some cases (e.g., protostomes), a spiral pattern is observed in the first few cell divisions, whereas in other cases (e.g., deuterostomes), the blastomeres (early cells) assume a radial configuration.

Besides having different types of cleavage, the embryonic cells of protostomes and deuterostomes have different potentials for future development. Animals that have a radial cleavage pattern (deuterostomes) have blastomeres that can potentially form all tissue types (i.e., their developmental fate is not predetermined); hence, this type of early development is termed indeterminate development. On the other hand, those animals with spiral patterns (protostomes) have blastomeres whose developmental fate is determined very early; hence, their development is termed determinate development.

Figure. 7(Click image to enlarge)

Totipotency is the capability of certain embryonic cells to form any type of adult cell. Hence, the early embryonic cells (blastomeres) of deuterostomes can be totipotent. Totipotency is required for two individuals to develop from a single zygote that is split in half (e.g., identical twins, also called monozygotic twins). Totipotency is typically lost during early embryo development, but scientists can reprogram some cells to become totipotent once again in the laboratory.

Dolly the sheep was cloned using the nucleus from a mammary (breast) cell that was rendered totipotent in the lab.

Figure. 8(Click image to enlarge)

Stem Cells and Development

In the early deuterostome embryo, each cell has the potential to form a complete embryo, i.e., they are totipotent. As development proceeds some embryonic cells begin expressing new genes and at the same time they lose the ability to form all possible cell types, but they still retain the ability to differentiate into a subset of cells; these cells are said to be pluripotent. In the popular press, both totipotent and pluripotent cells can be referred to as stem cells. Understanding how stem cell maintain their totipotency (and how their derivitives lose this character) is an important area of research in developmental biology. Medical researchers are also interested in this area as it may provide new treatment options for patients that have irrepairable damage to certain tissues. However, ethical questions have arisen regarding the use of human embryos to obtain totipotent stem cells and consequently research, particularly in in this country, has limited the widespread acceptance of this research. In 2007, researchers reported on the production of pluripotent cells from highly differentiated human fibroblasts (a lighly differentated cell) and this approach holds the promise to further advance stem cell research without the need of human embryos.

From Zygote to Induction

The figure on the right illustrates how the first collection of cells (blastomeres) form a hollow ball, the blastula. However, blastulae aren't always hollow spheres; they can be flattened (e.g., the blastulae of birds and mammals). From this hollow embryonic stage, cells start migrating into the interior and simultaneously begin to differentiate. This process is called gastrulation. During gastrulation, cells change their developmental pathway. The gastrula stage, in which tissue types arise, is only observed in the eumetazoans.

Figure. 9 (Click image to enlarge)

At the conclusion of gastrulation, the endoderm, ectoderm and mesoderm appear. In addition, the archenteron and blastopore arise. The archenteron is a clear pouch of cells created by the invagination during gastrulation, and the blastopore is the opening to the pouch. (In the simplest scenario, think of pushing in on a balloon with your finger; the area that your finger tip forms (the pouch) represents the archenteron, and its opening represents the blastopore.) The blastopore's fate is determined by the type of organism. If the blastopore gives rise to a mouth, it is a protostome. If the blastopore gives rise to an anus, it is a deuterostome. The blastocoel is the first cavity that forms. (Once again, don't confuse "cavity" with "gut.") The archenteron is the second cavity that forms; the archenteron is the cavity that will give rise to the gut in coelomates.

In the figure above, the cells that migrate inward are a different color; they express a different set of genes. Gastrulation, the movement of surface cells inward, is accompanied by induction (the process by which certain embryonic cells trigger the differentiation of other embryonic cells).


This tutorial began our discussion of Kingdom Animalia. Animals have been around for at least 700 million years, and during this time they have diversified to a remarkable degree. Likely, the ancestor of all animals was a colonial protist. In examining the phylogenetic tree, keep in mind that there are major dichotomous characters that occur at each branch point. In many instances these characters are based on the development of animals.

Eumetazoan development (as with other sexually reproducing, multicellular eukaryotes) begins with fertilization. The zygote then undergoes a series of cell divisions to produce a mass of cells that has some hollow character (the parazoans do not follow this type of embryology). The first tissue to form is the blastoderm, and this young tissue surrounds the blastocoel. In the next step (gastrulation), cells migrate inward and begin to differentiate into endoderm. Thus, gastrulation marks the embryonic stage where additional tissue types form. In the case of diploblastic animals, there are only two tissue types (ectoderm and endoderm), whereas triploblastic animals have three tissue types (ectoderm, mesoderm, and endoderm). During gastrulation, the primitive gut also forms from the archenteron.

Depending on the species, a second body cavity (coelom) can form. Thus, animals can be characterized as lacking a second body cavity (acoelomates), having a coelom that is incompletely lined with mesoderm (psuedocoelomates), or having a coelom that is completely lined with mesoderm (coelomates). As we survey the various phyla of Kingdom Animalia, keep in mind the relationship between these major character traits and their relationship with animal development.