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Animals II - Parazoa, Radiata, and Acoelomates

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Introduction and Goals

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 branch points in the phylogenetic tree of animals. By the end of this tutorial you should have a basic understanding of:

  • 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

Performance objectives:

  • 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.  Animal Diversity and Body Plans. (Click image to enlarge)

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 is Phylum Porifera, which includes the sponges (Fig. 2). As mentioned in Tutorial 18, some scientists still question whether sponges are really animals and, if so, are they really 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. Given that they appear to be a mass of relatively unspecialized cells, one might wonder if sponges are actually individuals or colonies of individuals. There is evidence that supports both choices.

Figure 2. A sponge. (Click image to enlarge)

Sponges: Structure


Figure 3.  A drawing of sponges. (Click image to enlarge)

Figure 4.  Sponge anatomy. (Click image to enlarge)

Sponges are asymmetrical; they do not have a plane that divides them into mirror images (Fig. 3). 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. 4). 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.

Sponges: Development

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. 5). 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 5. Sponge Development. (Click image to enlarge)

While we mainly use synthetic sponges, natural sponges are still sold for household and artistic use. Figure 6 shows how sponges are sold at a market in Crete, Greece

Figure 6. 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. 7). The radiata include organisms that have a radial morphology. The radiata include 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 7.  Animal Diversity and Body Plans. (Click image to enlarge)

All cnidarians have true tissues and are members of the Eumetazoa.They have one of two characteristic body plans (Fig. 9) and Hox genes are found in their genome.  They all obtain and digest nutrients in an organized cellular manner, and many types move through the environment using muscle-like cells at the based of the epithelial (covering) cells. They have a nerve net that allows them to react more quickly to their environment. Cnidarians have a gastrovascular cavity that is used for both digestion and excretion. The mesoglea is a jelly-like substance between the two tissue layers that provides support.

Figure 8. Corals. (Click image to enlarge)

The cnidaria have two general body plans: the polyp and the medusa (Fig. 9). The polyp form is often sessile, anchored to a substrate. The medusa form is mobile. Recall from our last tutorial, that the radiata have only two embryonic tissue layers. Specifically, they lack mesoderm (the tissue that gives rise to structures, including muscles, in triploblastic organisms). As a consequence, they lack the more sophisticated movement seen in triploblastic organisms, but they do have muscle-like cells that enable them to move by contraction. Note that the mouth and anus are actually a single structure.

Figure 9. General Cnidaria Body Plans: Polyp and Medusa. (Click image to enlarge)

Cnidarians also have cnidocytes (specialized cells that function in defense and the capture of prey); cnidocytes contain organelles called cnidae, which are able to evert. Cnidae that sting are called nematocysts (Fig. 10). These nematocysts can immobilize fish for capture, and they can also be used for defense.

Figure 10. Cnidocytes contain nematocysts, used to capture prey, and in defense. (Click image to enlarge)

Cnidarian Classes

There are four classes in the Phylum Cnidaria. You do not need to know the class names, but you should understand the diversity of body types and ecology seen among the members of this phylum.

Class Hydrozoa includes hydras (Fig. 11), and the infamous Portuguese Man-of-War, an organism noted for its potent sting, in the medusa form. Most hydrozoans exist in the polyp and medusa forms. For example, some hydras exist as an asexually reproducing polyp that alternates with a sexually reproducing medusa form (Fig. 12).


Figure 11.  Hydra oligactis, a freshwater hydra found in North America, in the polyp form.

Figure 12.  The life cycle of Obelia, a hydrozoan. (Click to enlarge)

Class Scyphozoa includes the jellies (or jellyfish), which exist predominantly in the medusa form (Fig 13). Class Cubozoa are the box jellies, which have a box-shaped medusa form. Some species in this group are among the most venomous organisms in the world and their stings can be fatal to humans.

Figure 13. A jelly. (Click image to enlarge)

Class Anthozoa includes sea anemones, corals (Fig. 14), and sea fans. Anthozoans exist only in the polyp form. In our discussion of Protist diversity (Tutorial 14), we addressed the symbioses between reef-building corals and the dinoflagellates. Recall that in these relationships, corals provide housing and protection for the protists, and the dinoflagellates provide food for the corals.

Members of all of the cnidarian classes can respond to external stimuli and can use stinging nematocysts for prey capture and defense. 

Figure 14. Anthozoans - the corals. (Click to enlarge)


We now turn our attention to those animals that have true tissues and bilateral symmetry, the bilateria. Recall from the previous tutorial 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 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 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 future 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. 15). 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.

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. S; msoc| ilcxIbR-SA'>Bilateria include acoelomates, pseudocoelomates, and coelomates. Acoelomates will be addressed in this tutorial, and the other two will be covered in future tutorials.

Figure 15. Overview of Animal Diversity and Body Plans. (Click to enlarge)

Pseudocoelomates Versus Coelomates

Figure 16 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 16. 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. The phylum Platyhelminthes is divided into four classes.

Members of the class Turbellaria include carnivorous flatworms (e.g., planarians). Cephalization occurs in the form of eyespots and paired ganglia, as well as an actual nervous system. These features are characteristic of the level of complexity observed in the bilateria. This class consists predominantly of free-living (nonparasitic) representatives (Fig. 17).  Many planarians are capable of regenerating a complete body from a very small fragment if they are cut into pieces.

Figure 17.  A planarian.

Members of the class Monogenea are all parasitic (as is the Polystomoides, Fig. 18). Six suckers are used to attach to and suck digested material from their hosts.

Figure 18. Polystomoides sp. Note the suckers used by this parasite to attach to its host.

Members of the class Trematoda are also parasitic. Some trematodes 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. 19).


Figure 19. A species of trematode.

Members of the class Cestoidea are also parasitic, and include the tapeworms. Tapeworms consist of a scolex (head), which has hooks for attaching to their host and suckers for extracting food (Fig. 20). The majority of their body is a series of proglottids, which basically are repeating units packed with sexual organs. Taenia serialis (Fig. 20) was recovered from a dog; one can see the hooks in the center of its head, surrounded by four suckers.

Figure 20. A Tapeworm. Note the scolex, which includes hooks for attachment to hosts

Platyhelminthes and Disease

The life cycle of Schistosoma mansoni, from the phylum Platyhelminthes, is complicated, involving multiple symbioses. Figure 21 shows a copulating pair of male and female Schistosoma (blood flukes). Sexual reproduction occurs inside a vertebrate host (e.g., a human).

Figure 21. Schistosoma mansoni. A copulating pair of male and female blood flukes

Figure 22 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. 23).

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 22. Life cycle of Schistosoma mansoni. (Click to enlarge)

Figure 23. 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 (Figure 24). 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 24. Person Suffering from Schistosomiasis. (Click image to enlarge)

People who work in or around, freshwater habitats of snails contaminated with human feces (Fig. 25) 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 25. A Potential Schistosomiasis Breeding Ground.  (Click image to enlarge)


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:

  • acoelomate
  • amoebocyte
  • anterior
  • coelomate
  • cephalization
  • choanocyte
  • cnidae
  • cnidarian
  • cnidocyte
  • flatworm
  • ganglion (pl. ganglia)
  • gastrovascular cavity
  • medusa
  • mesoglea
  • nematocyst
  • osculum (pl. oscula)
  • planaria
  • polyp
  • posterior
  • proglottid
  • pseudocoelomate
  • scolex
  • spongocoel
  • tapeworm
  • trematode

Case Study for Animals II

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? 

Now that you have read this tutorial and worked through the case study, go to ANGEL and take the tutorial quiz to test your understanding.  Questions?  Either send your instructor a message through ANGEL or attend a weekly review session (the times and places are posted on ANGEL).