Introduction and Learning Objectives
This tutorial will begin our discussion of Kingdom Animalia. First, we will discuss what makes an organism an animal. Then, we will focus on some major innovations in the evolution of animals. Then we will discussion animal development (from a single-cell zygote through a specialized embryo). This tutorial will begin our discussion of specific phyla within the kingdom Animalia, beginning with the parazoans and continuing through the acoelomates. As we introduce new topics in animal classification, keep in mind the phylogenetic tree of animals (Fig. 1). Also, think about how form (morphology) relates to function. That is, how does an animal's shape and organization affect what it is able to do? This tutorial will provide a more detailed look at the first three major branching points in the phylogenetic tree of animals. The following two tutorials will continue a discussion of animal diversity. By the end of this tutorial you should have an understanding of:
- The general characteristics of all animals
- The origin and early evolution of animals
- The major early branching points in the phylogeny of animal evolution
- The basic steps involved in animal development
- Characteristics of organisms in the phyla Porifera, Cnidaria, and Platyhelminthes
The different types of body cavities found in bilateral animals
- How certain flatworms can cause disease in humans
- Summarize the general characteristics of animals
- Review the evolutionary history of animals and their relationship to protists
- Define tissues, symmetry, cephalization
- Describe the early stages of animal development, including tissue and gut formation
- Differentiate between radiata and bilateria, diploblasts and triploblasts
- Discuss the role of Hox genes in animal development
- Discuss the reason for the basal position of sponges in the animal phylogenetic tree and the characteristics of sponges
- Explain the characteristics of the Radiata
- Diagram the three different body cavity types found in the bilateria
- Describe the characteristics of the flatworms and some examples of diseases they cause
Figure 1. Phylogenetic tree showing the relationships among the major lineages of animals. This tree is based on morphological and developmental data. (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 anything living. Why are these diverse organisms (Fig. 2) all classified as animals?
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.
Figure 2. Animal Diversity in a Saltwater Aquarium. (Click image to enlarge)
Figure 3. Sponge Development. (Click image to enlarge)
Animals are ingestive heterotrophs;, they ingest nutrients, bringing them into their bodies for digestion (Fig. 4). Unlike plants, which store their food as starch, animals store their food as glycogen.
Most animals have muscle tissue and nervous tissue. These tissues can range from being simple (as seen in members of the radiata) to being highly complex (as seen in vertebrates). The coordination between muscle tissue and nervous tissue can result in very specialized movements (e.g., getting food, pursuing a mate, avoiding a predator).
Figure 4. Cheetahs preparing to ingest prey. (Click image to enlarge)
The Origin of Animals
The first animals likely originated from a colonial protist similar to choanoflagellates (Fig. 5). Choanoflagellates share some basic feature with animals, the parazoa in particular. This colony of individual cells is anchored to a substrate, and each flagellum sweeps food into a cell for ingestion. The cell can then transfer the food to other cells. While this organism shows little specialization, it displays a very primitive form of multicellularity and ingestive heterotrophy.
Molecular data also place choanoflagellates the group of protists most closely related to animals. Animals first appear in the fossil record approximately 750 million years ago, but molecular data suggest that animals evolved much earlier. Read about work that was conducted at Penn State University to determine when various taxa evolved in the link at the end of this paragraph. Be prepared to answer questions about their methods and conclusions in the tutorial quiz. Large Gene Study Questions Cambrian Explosion.
Figure 5. Drawing of a Choanoflagellate. (Click image to enlarge)
Animal Evolution: The Big Picture
This figure (Fig. 6) 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.
Figure 6. Overview of Animal Diversity and Body Plans. (Click image to enlarge)
Parazoa Versus Eumetazoa
The first dichotomous branching point of the phylogenetic tree of Kingdom Animalia (Fig. 6) 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 (Fig. 7). 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).
Figure 7. Sponge Development. (Click image to enlarge)
The Eumetazoans: Radial Versus Bilateral Symmetry
The second dichotomous branch point in the Animal phylogenetic tree (Fig. 8) 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.
Figure 8. Overview of Animal Diversity and Body Plans. (Click image to enlarge)
Protostomes Versus Deuterostomes
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 (Fig 8). 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.
In the kingdom Animalia, organisms are primarily grouped according to their development. Early development is conserved across the members of this kingdom. This means that two animals can appear to be very similar when in an early 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 embryonic cells of protostomes and deuterostomes have different potentials for future development (Figure 10). Protostomes have blastomeres whose developmental fate is determined very early; hence, their development is termed determinate 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.
Totipotency is the capability of certain embryonic cells to form any type of adult cell. 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.
Figure 10. Protostome Versus Deuterostome Development.(Click image to enlarge)
From Zygote to Induction
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.(Fig. 10A).
Figure 11 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; the cells that migrate inward express a different set of genes. This sets up the first two tissue layers, ectoderm and endoderm. Gastrulation, this movement of surface cells inward, is accompanied by induction (the process by which certain embryonic cells trigger the differentiation of other embryonic cells). During gastrulation, cells change their developmental pathway. The gastrula stage, in which tissue types arise, is only observed in the eumetazoans.
Figure 11. Early Embryonic Development Through Gastrulation. (Click to enlarge)
At the conclusion of gastrulation, the endoderm and ectoderm have been defined, and the mesoderm then forms (Fig. 10B). In addition, the archenteron and blastopore arise. The archenteron is a pouch of cells created by invagination during gastrulation, and the blastopore is the opening to the pouch. (In the simplest scenario, think of pushing into a lump of clay 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 (Fig. 10C). 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 gut forms from the archenteron.
Hox genes determine the body pattern in animals
Hox genes are found in animals and determine the pattern formation of the body during the development of the embryo. These genes code for transcription factors, which control the expression of other genes. In animals, these genes are found in a linear sequence along the chromosome, starting with the genes that affect the structure at the head of the organism, and moving down the body (Fig.12).
These genes were first discovered in the fruit fly (an invertebrate model organism) which has one cluster of hox genes, and are now known to be in all animals. They are also well-studied in the mouse (a vertebrate model organism) which has four clusters of hox genes (humans also have four clusters). These additional clusters have taken on new functions, including determining the patterning of the limbs. Mutations in these genes can lead to a change in the postion of body parts, and the results include flies that have legs where their antennae should be!
Figure 12. Drosophila melanogaster (top) and Mus musculus (bottom). The color shows the location of the expression of a particular gene in the hox cluster (only one of the four mouse clusters is shown). (http://www.nature.com/nrc/journal/v2/n10/box/nrc907_BX2.html*)
Parazoa: The Phylum Porifera (Sponges)
The first dichotomous branch point in the phylogenetic tree of animals distinguishes between the parazoans and the eumetazoans; organisms lacking true tissues versus those that have truly specialized tissues.The first phylum we'll discuss are the parazoans of Phylum Porifera, the sponges (Fig. 13).
Some scientists still question whether sponges are really animals and, if so, are they individuals or colonies of individuals? While sponges are composed of a loose collection of cells, they lack the true tissue-level organization that is characteristic of eumetazoans. There is evidence that supports both choices.
Figure 13. A sponge. (Click image to enlarge)
Figure 14. A drawing of sponges. (Click image to enlarge)
Figure 15. Sponge anatomy. (Click image to enlarge)
Sponges are asymmetrical; they do not have a plane that divides them into mirror images (Fig. 14). They also do not have true Hox-genes, but do have Hox-like genes that may be involved in body pattern formation.
Sponges have an epidermis composed of tightly packed cells, underneath which lies a gelatinous matrix and a few specialized cell types that surround a central cavity termed the spongocoel (Fig. 15). The spongocoel is connected to the outside via an opening called the osculum. The spongocoel is lined with choanocytes, which are cells that have a central flagellum and a sticky collar that surrounds the flagellum. Which organisms do these cells look like? Water is drawn into the spongocoel through the pores, and food particles in the water may pass the sponge's choanocytes. The flagellum of a choanocyte pulls in food particles, which get stuck in the sticky mucus of the collar and are picked up by amoebocytes. Amoebocytes are mobile and can transport nutrients throughout the body of the sponge. Water then exits the sponge through the osculum.
A sponge has an embryonic form similar to a blastula because it is hollow. One-half of this embryonic form is flagellated, hence the embryo is free-swimming (Fig. 16). This hollow, half-flagellated ball will eventually settle and become stuck to a substrate. The flagellated half will invert, and the point where it inverts will become the osculum. The space created during inversion will become the spongocoel. Note that the adult sponge is sessile, whereas the embryonic form is motile.
Figure 16. Sponge Development. (Click image to enlarge)
While we mainly use synthetic sponges, natural sponges are still sold for household and artistic use. Figure 17 shows how sponges are sold at a market in Crete, Greece
Figure 17. Sponges - for sale in a Greek market. (Click image to enlarge)
Radiata: The Phylum Cnidaria
The second dichotomous branch point of the animal phylogenetic tree distinguishes between the radiata and the bilateria (Fig. 17). The radiata include organisms that have a radial morphology, including Phylum Cnidaria and and Phylum Ctenophora (in the past these two groups were place in the Phylum Coelenterata, but we now know that they are separate lineages). Examples of cnidarians include jellies (often called jellyfish), corals (Fig. 8), and sea anemones. Some cnidarians are bioluminescent (radiate light), and some (e.g., the Portuguese Man-of-War) can sting.
Figure 17. Animal Diversity and Body Plans. (Click image to enlarge)
Figure 18. General Cnidaria Body Plans: Polyp and Medusa. (Click image to enlarge)
Members of the Cnidaria include the corals (Fig 19), hydras (Fig. 20), jellies (Fig. 21) and sea anemones. Most species are marine although some hydra are found in freshwater environments.
Figure 19. Corals. (Click image to enlarge)
Figure 20. Hydra oligactis, a freshwater hydra found in North America, http://en.wikipedia.org/wiki/Hydra_oligactis
Figure 21. A jelly. (Click image to enlarge)
Figure 22. Anthozoans - the corals. (Click to enlarge)
Figure 23. Cnidocytes contain nematocysts, used to capture prey, and in defense. (Click image to enlarge)
A Real Life Example: Coral Bleaching
Coral bleaching is the loss of color of coral reefs due to the loss of zoothanthellae algae that live symbiotically within anthozoan corals. Bleaching is a worldwide problem and likely involves many factors. One area of study focuses on how the global warming of ocean waters is creating an unfavorable environment for anthozoan coral-builders and their symbionts. In a healthy coral community the zooxanthellae actively photosynthesize and produce more anabolic products than their own catabolic pathways consume; the excess anabolic products are then passed to the anthozoan coral builders which use the products for activities that include the calcification required to build a coral reef. It has been noted that when the temperature is elevated, Photosystem II can become damaged at high light levels. The causes of the process of bleaching are still under debate and it is not clear whether the zooxanthellae leave the anthozoan on their own, or whether the anthozoan actively rejects the symbiont.
- Describe the symbiotic relationship between the zooxanthellae and the anthozoa that live in waters in which they co-evolved and compare this to the symbiotic relationship that now exists in areas experiencing bleaching.
- Based upon this information would you predict that coral bleaching increases the evolutionary fitness of the anthozoan host? Why or why not?
We now turn our attention to those animals that have true tissues and bilateral symmetry, the bilateria. Recall that the bilateria have a single plane of symmetry. In tandem with this single plane of symmetry is the development of advanced sensory material in the anterior (front) part of the body. The eyes of planaria, an acoelomate, lack the resolution of our own eyes, however, they do detect light. They are connected to a primitive brain (simple brains that contain a low number of neurons are sometimes referred to as ganglia.) The eyes are positioned to perceive new surroundings as the organism moves into new areas. This trend toward a concentration of sensory organs and nervous tissue at the anterior end of an animal is termed cephalization. Remember, the bilateria are triploblastic, possessing a third embryonic tissue layer (mesoderm).
Bilateria include acoelomates, pseudocoelomates, and coelomates. Acoelomates will be addressed in this tutorial, and the other two will be covered in the following tutorials.
Acoelomates versus Coelomates
The third major bifurcation of the animal phylogenetic tree distinguishes animals by whether or not they have a body cavity (Fig. 24). 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 tutorial). 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.
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.
Figure 24. Overview of Animal Diversity and Body Plans. (Click to enlarge)
Pseudocoelomates Versus Coelomates
Figure 25 illustrates the body plans of the bilateria. Note the acoelomate, pseudocoelomate and coelomate conditions. Remember, acoelomates do not have a fluid-filled body cavity, while both psuedocoelomates and coelomates have a body fluid-filled body cavity. What advantages does this cavity provide? Do you see that pseudocoelomates (e.g., nematodes) have a body cavity that is only partially lined by mesoderm-derived tissue? In contrast, the coelomates have a body cavity that is completely lined with mesoderm. Keep in mind that mesoderm gives rise to muscle (as well as other organs).
Figure 25. Bilateria Body Plans. (Click to enlarge)
Acoelomates: The Phylum Platyhelminthes (Flatworms)
In our discussion of the bilateria, we begin with those organisms that do not have a body cavity (the acoelomates). These organisms do not have a fluid-filled internal body cavity, but instead, have a relatively solid body mass. Members of the Platyhelminthes include the carnivorous flatworms(e.g., planarians, Fig. 26). Cephalization occurs in the form of eyespots and paired ganglia, as well as an actual nervous system. Many planarians are capable of regenerating a complete body from a very small fragment if they are cut into pieces.
Tapeworms are parasites that consist of a scolex (head), which has hooks for attaching to their host and suckers for extracting food (Fig. 27). This Taenia serialis was recovered from a dog; you can see the hooks in the center of its head, surrounded by four suckers.
The flukes are also parasitic. Some flukes exhibit very complex life cycles. Examples include the various species of blood flukes in the family Schistosoma, and the liver fluke depicted C. sinensis (Fig. 28).
Figure 26. A planarian. http://web.mit.edu/neuro/planaria.html
Figure 27. A Tapeworm. Note the scolex, which includes hooks for attachment to hosts
Figure 28. A species of fluke.
Platyhelminthes and Disease
The life cycle of Schistosoma mansoni, a fluke in the Phylum Platyhelminthes, is complicated, involving multiple symbioses. Figure 28 shows a copulating pair of male and female Schistosoma. Sexual reproduction occurs inside a vertebrate host (e.g., a human).
Figure 28. Schistosoma mansoni. A copulating pair of male and female blood flukes
Figure 29 illustrates the complex life cycle of Schistosoma mansoni. Fertilized eggs are eliminated in the feces of the first host. The larvae that emerge parasitize a second host, a snail (Fig. 30).
The flukes reproduce asexually within the snails, and their second-stage larvae emerge to infect yet another vertebrate host. The larva exhibit chemotaxis, attracted to vertebrates by the chemicals on their skin surface. After attaching to the skin, the larvae break down skin protein and enter the bloodstream through a venule (located between a capillary and its vein). Once in the bloodstream, the parasite migrates to the lungs, and then the liver. The entire process takes about ten days.
Figure 29. Life cycle of Schistosoma mansoni. (Click to enlarge)
Figure 30. A snail host. Blood flukes reproduce asexually within snails. (Click image to enlarge)
People who suffer from schistosomiasis exhibit various symptoms, including a distended abdomen (Fig.31). Other symptoms include pain, extreme diarrhea, and developmental impairment in children. Though the disease has a low mortality rate, but its morbidity is such that schistosomiasis ranks as the second most socioeconomically-devastating disease in the world, after malaria.
Figure 31. Person Suffering from Schistosomiasis. (Click image to enlarge)
People who work in or around, freshwater habitats of snails contaminated with human feces (Fig. 32) are at risk for harboring Schistosoma and contracting schistosomiasis. The infection can be treated with an annual dose of praziquantel at a cost of about a dollar per person.
Figure 32. A Potential Schistosomiasis Breeding Ground. (Click image to enlarge)
This tutorial began our discussion of Kingdom Animalia. Animals have been around for over a billion years, and during this time they have diversified to a remarkable degree. The ancestor of all animals was most likely a colonial protest similar to a choanoflagellate. In examining the animal phylogenetic tree, keep in mind that there are major dichotomous characters that occur at each branch point, including the presence of tissues, the number of tissue layers, and the symmetry of the organism. In some cases 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. Hox genes are found in all animals and determine the patterning of the body during development.
As we survey the various phyla of Kingdom Animalia, keep in mind the relationship between these major characters and animal development. This tutorial examined the early branches in the Kingdom Animalia. Be sure that you understand the rationale behind the taxonomic scheme for animals. For example, it is important for you to know what a coelomate is, what the coelom represents, and how the coelom arises.
The Phylum Porifera was discussed. These animals can be considered individuals and colonies because they have features of both. In fact, there are some biologists who argue that they really aren't animals; however, they do have animal characteristics (they are multicellular, motile, ingestive heterotrophs), which is why they are placed in the kingdom Animalia.
Phylum Cnidaria is comprised of simple animals, including jellies and corals. These animals do have true tissues, however, they possess only two embryonic tissues; hence, they have a diploblastic mode of development. The major body forms observed in this phylum are the polyp and the medusa. In many cnidarians, these forms alternate during the life cycle, but in Class Hydrozoa the polyp is prominent, whereas in Class Scyphozoa the medusa is prominent. Members of Class Anthozoa do not have a medusa stage (this class includes the coral reef-building animals that form symbioses with dinoflagellates). Think about these differences in terms of evolution because small changes in the expression of developmental genes can control when, and for how long, a given form is represented. Within Phylum Cnidaria, there are differences in the timing of the polyp versus the medusa stage.
The bilateria were introduced in this tutorial. Keep in mind, the character trait bilateral symmetry is observed in animals that actively move through their environments. Bilaterally symmetrical animals not only have a single plane of symmetry, their sensory and cephalic areas are usually cephalized, placed toward the anterior part of the animal. Bilateral animals can be characterized as lacking a body cavity (acoelomates). Those that have a body cavity, coelom, may have one that is incompletely lined with mesoderm (pseudocoelomates), or one that is completely lined with mesoderm (coelomates).
The Phylum Platyhelminthes is composed of animals that are commonly referred to as flatworms. These animals are triploblastic and show an organ level of complexity. Although many members of this phylum are free-living, some are parasitic and cause major health problems in some parts of the world.
After reading this tutorial, you should have a working knowledge of the following terms:
Questions? Send your instructor a message through ANGEL!