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Animals III - Pseudocoelomates and Protostome Coelomates

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Introduction and Goals

This tutorial continues our discussion of the major features of animal evolution (Fig. 1), concentrating on the bilateria. Previously we introduced three phyla: Porifera, Cnidaria, and Platyhelminthes. These groupings encompass representatives of the parazoa, radiata, and bilateria, respectively. We will begin by distinguishing among the organisms with body cavities, the pseudocoelomates and true coelomates. By the end of this tutorial you should have a better understanding of:

  • How specific organisms coincide with the major branch points in the phylogenetic tree in the evolution of animals
  • Characteristics of organisms in Phyla Nematoda, Mollusca, and Annelida
  • How certain roundworms cause diseases in humans

Performance objectives:

  • Identify the two groups of pseudocoelomates, their general characteristics and the human impacts of nematodes
  • Compare and contrast the two groups of animals with true coelems, the protostomes and deuterostomes, including the differences in development seen in these two groups
  • Discuss the characteristics of the organisms that are members of the Mollusca
  • Describe the characteristics of the organisms that are members of the Annelida

Figure 1. Animal Diversity and Body Plans. (Click image to enlarge)

Coelomates and Pseudocoelomates

Figure 2 illustrates the three basic body plans encountered in the bilateria: the acoelomates (e.g., flatworms, flukes, and tapeworms), the pseudocoelomates (e.g., roundworms, pinworms, hookworms and rotifers), and the coelomates (e.g., snails, clams, octopuses, earthworms, and leeches). Recall, pseudocoelomates differ from coelomates in that their body cavities are not completely lined by mesoderm-derived tissue.

Figure 2. Bilateria. Three Basic Body Plans.  (Click image to enlarge)

Pseudocoelomates: The Phylum Nematoda

Roundworms are pseudocoelomates that belong to the phylum Nematoda (Fig. 3). This extremely diverse phylum includes some highly beneficial free-living soil worms, as well as some notorious pests and parasites. The highly pressurized pseudocoelom of these animals can function as a hydrostatic skeleton, which can be useful in drilling through soil or a host's body tissue. Annelids also have hydrostatic skeletons.

Figure 3. Animal Diversity and Body Plans. (Click image to enlarge)

These giant kidney worms (Fig. 4) are able to parasitize many mammals, including humans. The female worms can grow up to 5 feet long. The worms live in the kidney of a vertebrate host, and their eggs leave the body in the host's urine. From there, the eggs might be consumed by an aquatic worm (from the phylum Annelida). Juvenile kidney worms can be transferred to fish or amphibians; if these secondary hosts are consumed by a vertebrate, the infection cycle is completed.


Figure 4.  Giant Kidney Worm - a nematode.  (Click image to enlarge)

Given how large these worms can become, it is not surprising that these parasites can cause extensive damage. Note how shrunken the asterisked kidney is, and also note the worms occupying the kidney in the yellow square (Fig. 5). The accompanying loss of kidney function can be deadly if the parasites are not detected and removed surgically.


Figure 5.  Kidney occupied by Giant Kidney Worms. (Click image to enlarge)

It is estimated that 25% of the world's population is infected with these roundworms (Ascaris lumbricoides). This particular mass of worms (Fig. 6) was passed from the intestine of a human. The ruler at the bottom is approximately 1.5 inches long. 


Figure 6.  Ascaris lumbricoides.  Roundworms that can infect the human intestines.   (Click image to enlarge)

Other nematodes include pinworms, hookworms, and Trichinella spiralis (the causative agent of trichinosis). This hookworm mouth (Fig. 7) reveals how these worms are able to latch on to a host and penetrate the host's intestines and other organs. Symptoms of helminth infections include diarrhea, anemia, pneumonia, as well as physical and mental retardation. The World Health Organization (WHO) data suggest that 1/3 of the world’s population is at risk of infection, with one in six people already infected. 


Figure 7.  A hookworm's mouth.   (Click image to enlarge)

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Coelomates: Protostomes versus Deuterostomes

The true coelomates are often categorized as either protostomes or deuterostomes (Fig 9). The distinction is based on differences in their early cell cleavage, coelom formation, and the fate of the blastopore.

Figure 9. Animal Diversity and Body Plans. (Click image to enlarge)

As mentioned earlier (Tutorial 18), one of the main differences between these two groups is in the origin of their gut. As shown in Figure 10C, the mouth develops first in protostomes, whereas in deuterostomes the anus develops first (the mouth forms secondarily).  However, recent studies have shown that  while some groups of protostomes all have the mouth developing from the blastopore (the Platyhelminthes, Nematodes and Rotifers), in other groups it is most of the species (the Molluscs and Annelids), while only a few Arthropods show this pattern.  Those studies also show that while all deuterstomes (Echinoderms and Chordates) have the anus developing from the blastopore, so do most Arthropods. However, Arthropods show the protostome condition for both cleavage (Fig. 10A) and the development of the mesoderm (Fig. 10B). So, while some of these developmental distinctions may not be as clear cut as we once thought, they can still be useful for categorizing animal diversity. We'll begin our discussion with the coelomate protostomes in the phylum Mollusca.

Figure 10. Protostome Versus Deuterostome Development. (Click image to enlarge)

Protostome Coelomates: The Phylum Mollusca

Animals in the phylum Mollusca has both aquatic and terrestrial species, including clams, snails, octopuses, and sea slugs (Fig. 11).

Figure 11. A Sea Slug. A gastropod mollusk.  (Click image to enlarge)

Mollusks are characterized by their soft bodies, which are usually protected by a hard calcium carbonate shell. However, this shell can be highly reduced or completely absent in some representatives of the phylum.


Class Gastropoda (meaning "stomach foot") includes snails, slugs, sea slugs, and nudibranchs (Fig. 12). Most snails have a coiled shell, however, the shell is completely absent in nudibranchs. Gastropods have terrestrial, marine, and freshwater representatives. Terrestrial gastropods (e.g., snails) lack the gills that are characteristic of other mollusks.

Figure 12. A Nudibranch. A gastropod mollusk. (Click image to enlarge)

Class Bivalvia includes mussels, oysters, and clams. Bivalves have a reduced head, two hinged shells connected by strong adductor muscles, and a body that is highly modified to fit within these shells.

Figure 13. A Clam. A bivalve mollusk. (Click image to enlarge)

Class Cephalopoda includes octopuses, squid (Fig. 14), cuttlefish, and chambered nautiluses. Cephalopods have a head surrounded by tentacles, which can be used for locomotion and grasping prey. The shells (also called pens) of squid and cuttlefish are reduced and internal. Recent research suggests that cephalopods are extremely intelligent organisms, and British institutional research review boards have gone so far as to enact special ethical protections for squid and octopuses similar to those afforded to higher level vertebrates. For more information on cephalopod intelligence, check out this article by Dr. Jennifer Mather, a neuroscientist with the University of Lethbridge.

Figure 14. A Squid. A cephalopod mollusk. (Click image to enlarge)

The largest invertebrates (animals without backbones) are the giant squids, which are extremely elusive and typically only encountered washed-up on the shore. The largest, documented giant squid was 18 meters long. The individual in this photo is 9 feet long. Recently, a Shell Oil Company diving robot caught a large Magnapinna “large fin” squid on video. To see a video and accompanying article on this fascinating creature, click here.

Figure 15. A 9'-Long Giant Squid. A cephalopod mollusk. (Click image to enlarge)

The cuttlefish (Fig. 16), from the class Cephalopoda, is a significant food source in many cultures.  Its internal skeleton, called the cuttlebone, is fed to pet birds to provide them with a source of calcium and other minerals.

Figure 16. Courting Cuttlefish. Cephalopod mollusks. (Click image to enlarge)

Class Polyplacophora includes the chitons (right 0. These flat organisms are often observed adhering to rocks in the intertidal zone.

Figure 17. A Chiton. A mollusk from Class Polyplacophora. (Click image to enlarge)

Mollusk Anatomy

This drawing of a snail's body plan (Fig. 18) is representative of the general body plan of mollusks. The internal anatomy of a representative mollusk (a snail) reveals how much more complex these organisms are than representatives from any of the preceding Animalia phyla discussed. 

In general, the mollusk body plan consists of a muscular foot, a visceral mass, and a mantle. The foot is used for movement (especially in the gastropods) or as an anchor (as observed in chitons). The visceral mass houses most of the internal organs (e.g., the stomach, gonads, and heart). The mantle is the tissue layer that covers the visceral mass. In organisms that have shells, the mantle produces the shell. Underneath the mantle is a mouth at one end and a mantle cavity at the other. The mantle cavity houses the anus and gills.


Figure 18. General Mollusk Body Plan. (Click image to enlarge)

Mollusks characteristically have a radula (a rasping structure that is used to scrape food particles from hard surfaces). For example, the radula of mollusks cleans algae off of the surfaces in aquariums.

Visit the marine aquarium in the HUB on the University Park campus and look for the radula on the underside of the snails cleaning the walls of the tank.  

To get involved with care and maintenance of this 500-gallon saltwater aquarium, visit the Penn State Marine Science Society website.

Figure 19. A Marine Aquarium. Located in the HUB at Penn State Univ.  (Click image to enlarge)

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Protostome Coelomates: The Phylum Annelida

Animals in the phylum Annelida include the earthworms, leeches, and many marine worms. These protostome coelomates exhibit true segmentation. Segmentation refers to body plans that are divided into discrete units, which may be repeating or may each have a unique function in the body.

Figure 20.  Animal Diversity and Body Plans.  (Click image to enlarge)

It is easy to detect segmentation in earthworms. The segments are physically separated internally by thin sheets of mesoderm-derived tissue termed septa (singular: septum; recall, the term septa was also used to describe structures that separate cells in the hyphae of fungi, i.e., septate hyphae; Tutorial16). Because of these separations, earthworms can contract muscles in some segments without affecting the hydrostatic pressure in adjacent segments. This ability is highly advantageous for movement, and through coordinated contraction and expansion of segments, earthworms are able to burrow into the soil with ease.

Figure 21. An Earthworm. An annelid. (Click image to enlarge)

Figure 22 depicts some of the internal segmentation in an earthworm. The bodies of most annelids consist of a series of repeating segments, with repetition of organ systems (muscular, nervous, reproductive, circulatory, and excretory) in the segments. Therefore, most annelids do not capitalize on one of the main benefits of segmentation, the ability to specialize. Note, many segments have multiple hearts, and each segment has a pair of nephridia (an excretory organ). Also note how the longitudinal blood vessels, nerve cords, and digestive tract all run the length of the body, whereas the pumping vessels and ganglia are all arranged in segments. The next tutorial will discuss how more advanced animals show extreme specialization by exploiting segmentation.


Figure 22. Internal Segmentation in an Earthworm. (Click image to enlarge)

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This tutorial continued our discussion of the bilateria branch of the kingdom Animalia. When you think about this character (bilateral symmetry), keep in mind that it is observed in animals that actively move through their environment. Bilaterally symmetrical animals not only have a single plane of symmetry, but their sensory and cephalic areas are usually displaced toward the anterior end of the animal. There are two lineages of pseudocoelomates. The nematodes include free-living species as well as many species that are parasites of mammals, while the mostly free-living rotifers are an important part of the zooplankton.  The mollucs are very diverse morphologically, ranging from the shell-less slugs to the bivalves with two large shells that are joined at a hinge.  Some cephalopods show remarkable levels of intelligence for an invertebrate.  The annelids show true segmentation, although their segments do not show extensive specialization.



After reading this tutorial, you should have a working knowledge of the following terms:

  • annelid
  • bivalve
  • cephalopod
  • foot
  • gastropod
  • hydrostatic skeleton
  • invertebrate
  • mantle
  • mantle cavity
  • mollusk
  • nematode
  • nephridium (pl. nephridia)
  • radula
  • roundworm
  • segmentation
  • septa
  • visceral mass

Case Study for Animals III

Caenorhabditis elegans is a microscopic nematode worm that has been used extensively as a model organism in biology. In the laboratory they are an ideal model organism because they can be maintained in a Petri dish, they eat bacteria, and they can be frozen for extended periods, and when thawed, they are completely viable. They have a clear cuticle through which all of their cells can be seen. All adult C. elegans have exactly 959 somatic cells and the individual developmental path of each of these cells has been mapped.

C. elegans is useful in genetic studies because it has a relatively small genome with 5 autosomes and one pair of sex chromosomes (adult C. elegans are either hermaphrodites (XX) or males (X)). It is also relatively easy to disrupt the action of individual genes using a procedure known as RNA interference. In this process, the action of individual genes can be “turned off” by using small pieces of RNA (known as small interfering RNA or siRNA) to degrade a complementary piece of mRNA. By “turning off” a gene, researchers can learn more about the function of that gene.

  • C. elegans’ natural habitat is soils. What feature does C. elegans (and other nematodes) have that allow them to move through soils?
  • C. elegans is used as a model organism to study biological phenomena in humans (for example, aging, sleep, and disease).  Describe one characteristic of C. elegans that makes it a useful model for these phenomena.  Describe one drawback to using C. elegans as a model for human biology.
  • How many different chromosome combinations can a hermaphrodite C. elegans produce as result of independent assortment during meiosis?

Now that you have read this tutorial and worked through the case study, go to ANGEL and complete the tutorial practice problems to test your understanding.  Questions?  Either send your instructor a message through ANGEL or attend an online office hour (the times are posted on ANGEL).