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Animals II - Parazoa, Radiata, and Acoelomates

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You should have a working knowledge of the following terms:

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

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. 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:

  • How specific organisms coincide with the first three major branch points in the evolution of animals
  • Characteristics of organisms in the phyla Porifera, Cnidaria, and Platyhelminthes
  • Characteristics of organisms in the classes Hydrozoa, Scyphozoa, and Anthozoa from the phylum Cnidaria
  • How certain flatworms can cause disease in humans

Figure. 1(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. As mentioned in the first tutorial on animals, 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 actually 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(Click image to enlarge)

Sponges: Structure

Figure. 3(Click image to enlarge)

Figure. 4(Click image to enlarge)

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. 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. Water is drawn into the spongocoel through the osculum, 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.

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. 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(Click image to enlarge)

While we mainly use synthetic sponges, natural sponges are still sold for household use. The image on the right illustrates how sponges are sold at a market in Crete, Greece.

Figure. 6(Click image to enlarge)

Radiata: The Phylum Cnidaria

The second dichotomous branch point of the phylogenetic tree distinguishes between the radiata and the bilateria. The radiata include organisms that have a radial morphology. The radiata include Phylum Ctenophora and Phylum Cnidaria (sometimes referred to as Phylum Coelenterata). Examples of cnidarians include jellies (often called jellyfish), corals (image below on the right), and sea anemones. Some cnidarians are bioluminescent (radiate light), and some (e.g., the Portuguese Man-of-War) can sting.

Figure. 1 (Click image to enlarge)

All cnidarians are able to contract and expand for movement and for reproduction. Additionally, they all obtain and digest nutrients in an organized cellular manner. Hence, they all have true tissues and are placed in the Eumetazoa. In addition, cnidarians have an unsophisticated gastrovascular cavity.

Figure. (Click image to enlarge)

The cnidaria have two general body plans and these are depicted in the image on the right: the polyp and the medusa. The polyp form is sessile, anchored to a substrate. The medusa form is motile. Note that the mouth and anus are actually a single structure.

Figure. 8(Click image to enlarge)

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 like muscles in triploblastic organisms). As a consequence, they lack the level of sophisticated movement seen in triploblastic organisms.

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. These nematocysts (shown at the right) can immobilize fish for capture, and they can also be used for defense.

Figure. 9(Click image to enlarge)

Cnidarian Classes

There are three classes in the Phylum Cnidaria.

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

Class Scyphozoa includes the jellies (or jellyfish). Scyphozoans exist predominantly in the medusa form.

Figure. 10(Click image to enlarge)

Class Anthozoa includes sea anemones, corals, and sea fans. Anthozoans exist only in the polyp form. In our discussion of protist diversity, 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.

Figure. 11(Click image to enlarge)

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

Examine the life cycle of Obelia and note that the medusa form is that which produces gametes. Once fertilization occurs, the animal undergoes development and a free-swimming planular larva results. The planula settles and develops into a sessile polyp that can develop asexually via budding. Polyps can differentiate into what appears like a colony in that the distal appendages are specialized for feeding or reproduction. The reproductive polyp produces a medusa and the cycle continues.

Figure. 12(Click image to enlarge)


We now turn our attention to those animals that have true tissues and bilateral symmetry, the bilateria. Recall from the previous tutorial, 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 and nervous material 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: 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.

Members of the class Monogenea are all parasitic (as is the Polystomoides species depicted at the right). Six suckers are used to suck digested material from their hosts.

Figure. 13 (Click image to enlarge)

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 at the right.

Figure. 14(Click image to enlarge)

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. The majority of their body is actually a series of proglottids, which basically are repeating units packed with sexual organs. Taenia serialis (right) was recovered from a dog; one can see the hooks in the center of its head, surrounded by four suckers.

Figure. 15 (Click image to enlarge)

Platyhelminthes and Disease

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

Figure. 16(Click image to enlarge)

Fertilized eggs are eliminated in the feces of the first host. The larvae that emerge parasitize a second host, a snail.

Figure. 17(Click image to enlarge)

The flukes reproduce asexually within the snails, and their second-stage larvae emerge to infect yet another vertebrate host.

Figure. 18 (Click image to enlarge)

People who suffer from schistosomiasis exhibit various symptoms, including a distended abdomen (like that seen above). Other symptoms include pain and extreme diarrhea.

Figure. 19 (Click image to enlarge)

People that work in, or around, freshwater contaminated with human feces are at risk for harboring Schistosoma and contracting schistosomiasis.

Figure. 20 (Click image to enlarge)


This tutorial examined the more basal dichotomous branches in the kingdom Animalia. Be sure that you understand the rationale behind the taxonomic scheme for animals. It will be 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 the common jellyfishes. 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. Recall that we discussed the role of heterochrony in Tutorial 13 on macroevolution. Within Phylum Cnidaria, there are differences in the timing of the polyp versus the medusa stage.

The bilateria branch of Kingdom Animalia was also discussed. 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 displaced toward the anterior part of the animal.

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.