Introduction and Learning Objectives
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 groups include representatives of the parazoa, radiata, and bilateria, respectively. We will begin by distinguishing among the organisms with body cavities, the pseudocoelomates and true coelomates. We will then discuss representative lineages of the protostome coelomates, including molluscs, annelids and arthropods. As you progress through this tutorial, compare and contrast these protostome taxa. How are these groups of animals similar? What morphological and developmental patterns do they have in common? How do they differ? By the end of this tutorial you should have a better understanding of:
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 that pseudocoelomates differ from coelomates in that their body cavities are not completely lined by mesoderm-derived tissue, but instead have mesoderm only on one side, with endoderm on the other.
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 members of this group have very diverse ecologies. The fluid-filled pseudocoelom of these animals can function as a hydrostatic skeleton, which can be useful in drilling through soil or a host's body tissue. Another group that we will study, Annelids, also have hydrostatic skeletons. Rotifers are pseudocoelomates that are an important part of the zooplankton.
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 mammal, 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 surgically removed.
Figure 5. Kidney occupied by Giant Kidney Worms. (Click image to enlarge)
It is estimated that 25% of the world's human population is infected with these roundworms (Ascaris lumbricoides). This particular worm (Fig. 6) was passed from the intestine of a human. The ruler at the side 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)
Nematodes as Model Organisms
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?
Coelomates: Protostomes versus Deuterostomes
Figure 8. Animal Diversity and Body Plans. (Click image to enlarge)
One of the main differences between these two groups is in the origin of their gut. As shown in Figure 9C, 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. 9A) and the development of the mesoderm (Fig. 9B). 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 9. Protostome Versus Deuterostome Development. (Click image to enlarge)
Protostome Coelomates: The Phylum Mollusca
Phylum Mollusca has both aquatic and terrestrial species, and includes clams, snails, octopuses, and sea slugs. 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.
Figure 10. A Sea Slug. A gastropod mollusk. (Click image to enlarge)
Figure 11. A Nudibranch. A gastropod mollusk. (Click image to enlarge)
For example, the gastropods (meaning "stomach foot") include snails, slugs, sea slugs (Fig. 10), and nudibranchs (Fig. 11). There are terrestrial, marine and freshwater species in this group. Most snails have a coiled shell, however, the shell is completely absent in nudibranchs. Terrestrial gastropods (e.g., snails) lack the gills that are characteristic of other mollusks.
Bivalves (“two shells”) are another groups of molluscs that includes mussels, oysters, and clams. They have a reduced head, two hinged shells connected by strong muscles, and a body that is highly modified to fit within these shells.
Figure 12. A Clam. A bivalve mollusk. (Click image to enlarge)
The cephalopods (“head foot”) include octopuses, squid (Fig. 13), cuttlefish, and chambered nautiluses. The members of this group 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 13. A Squid. A cephalopod mollusk. (Click image to enlarge)
The largest living invertebrates (animals without backbones) are the giant squids (Fig. 14), 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 14. A 9'-Long Giant Squid. A cephalopod mollusk. (Click image to enlarge)
The internal anatomy of a representative mollusk (a snail) shown in Fig. 15 reveals how much more complex these organisms are than representatives from any of the preceding animal 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. 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 15. 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. Some snails use their radula to drill into the shells of other molluscs.
Visit the marine aquarium in the HUB (Fig. 16) 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 16. A Marine Aquarium. Located in the HUB at Penn State Univ. (Click image to enlarge)
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 17. Animal Diversity and Body Plans. (Click image to enlarge)
It is easy to detect segmentation in earthworms (Fig. 18). 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; Tutorial 12). 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 18. An Earthworm. An annelid. (Click image to enlarge)
Figure 19 shows 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. In contrast, some arthropods show extreme specialization by exploiting segmentation.
Figure 19. Internal Segmentation in an Earthworm. (Click image to enlarge)
In terms of numbers and the diversity of species, Phylum Arthropoda is the most successful group in Kingdom Animalia. Whether one looks on the land, in the air, or under the sea, they will find representatives from this phylum. Over one million arthropod species have been described to date; of these, about 400,000 are beetles (Fig. 20). Some researchers estimate that the number of arthropod species may exceed 10 million..
Figure 20. A beetle. An arthropod (member of the Insecta). (Click image to enlarge)
Characteristics of Arthropods
The extreme diversity observed in Phylum Arthropoda can be attributed to three main arthropod characteristics that have evolved into various forms to allow for adaptation to different environments: a hard chitinous exoskeleton, body segmentation, and jointed appendages (the word arthropod means jointed leg) (Fig. 21).
The arthropod body is covered by a cuticle composed primarily of the polysaccharide chitin (the same polymer used by fungi, as was mentioned in the Fungi I tutorial, but with a harder character; chitin was also described in the tutorial Carbon and Life). In crustaceans, the cuticle includes calcium carbonate, the same mineral used in mollusk shells. This exoskeleton demonstrates several features that are adaptations for life in a terrestrial environment. Its durability protects the animals from physical injury. It also provides structural support to the muscles that move the appendages. Lastly, the cuticle is waterproof and helps prevent desiccation in a dry terrestrial environment; the outer cuticle is covered by a thin layer of wax. One drawback of the exoskeleton is that it confines growth, but arthropods deal with this problem by periodically shedding their exoskeletons in a process known as molting. The cuticle also creates a barrier to gas exchange. While aquatic arthropods use gills, terrestrial arthropods “breathe” through a series of small holes in the body, spiracles, that lead to trachea, tubes that carry gases into and out of the tissues
Figure 21. A Crab. An arthropod (member of the Crustacea). (Click image to enlarge)
Arthropods are segmented, and generally there are distinct boundaries between the segments. For example, animals belonging to Class Insecta have three distinct segments: the head, the thorax (sometimes grouped together with the head as the cephalothorax), and the abdomen. The rhinoceros beetle (Fig. 22) has a cephalothorax and a large abdomen, while an ant has a distinct head and thorax. Typically, these different body regions have distinct functions and often contain various types of jointed appendages. Jointed appendages afford the animal with a greater degree of movement. In addition to locomotion, the appendages may be adapted for other functions (e.g., feeding, sensory perception, copulation, and defense).
Figure 22. A Rhinoceros Beetle. An arthropod (member of the Insecta) (Click image to enlarge)
Some jointed appendages, such as the swimming appendages of this candy cane shrimp (Fig. 23) are specialized through adaptation to different and diverse environments.
Figure 23. A Candy Cane Shrimp. An arthropod that is a crustacean. (Click image to enlarge)
Figure 24 shows the body segmentation (cephalothorax and abdomen) and specialized appendages of a representative arthropod, a lobster. In addition to segmentation, arthropods have an open circulatory system; their “blood” and internal organs are contained within a coelem, the hemocoel.
Figure 24. A Lobster. An arthropod that is a crustacean. (Click image to enlarge)
Phylum Arthropoda: Classification
The arthropods are traditionally divided into five subgroups or subphyla: trilobites (an exclusively marine group whose members all are extinct; the fossil record indicates that they were once the dominant subgroup), chelicerates, myriapods, crustaceans, and hexapods. As is the case for many groups of organisms, new molecular datasets are providing additional information on relationships. The groups presented here may change as future molecular data provide a clearer insight into arthropod relationships.
Figure 25. A Scorpion. An arthropod that is a chelicerate, and also an arachnid. (Click image to enlarge)
The majority of modern chelicerates (e.g., spiders, scorpions, ticks, and mites) are terrestrial arthropods belonging to Class Arachnida. As with the trilobites, most of the marine chelicerates are extinct, even though a few marine members (e.g., the horseshoe crab, members of a separate class) have survived. The arachnids (Figs 25 and 26) are best distinguished by their claw-like feeding appendages (chelicerae), from which this subgroup gets its name. Another characteristic of the chelicerates is the presence of two body segments (a cephalothorax and an abdomen). The cephalothorax has six pairs of appendages, including four pairs of walking legs, one pair of chelicerae, and one pair of pedipalps, which have either a feeding or sensory function. Chelicerates lack antennae and have simple eyes.
Figure 26. A tarantula, a type of spider. (Click image to enlarge)
The crustaceans make up the second largest arthropod subgroup. Extant species are mainly aquatic animals, although some terrestrial species (e.g., pill bugs and wood lice) are classified within this group. Animals in this class include crabs, lobsters, crayfish and shrimp, and they are the only arthropods with two pairs of antennae. Their appendages have two branches at the tips (see the crab claw in Fig. 9) and sometimes the term birame is used to describe members of this group. Segmentation is obvious and extensive in these animals. In contrast to the chelicerates, crustaceans have jaw-like mandibles and compound eyes. The fiddler crab (Fig. 27) is a typical crustacean.
The myriapods include the centipedes and millipedes. All of the members of this group are terrestrial, and they have long bodies with many segments, although the segments lack specialization. They have a single pair of antennae and usually have simple eyes (Fig. 28).
Figure 27. A Fiddler Crab. An arthropod that is a crustacean. (Click image to enlarge)
Figure 28. A millipede. An arthropod that is a myriapod. (Click image to enlarge)
The hexapoda (“six legs”), the largest extant subgroup, represent so many individual species that this subgroup accounts for the majority of all known (and probably most of the still undiscovered) animal species on the planet (Fig. 29). The hexapods have mandibles and compound eyes (as do the crustaceans). Uniquely, they have only one pair of sensory antennae and their appendages are unbranched or uniramous (the name unirames is sometimes used for this group). The hexapods include Class Insecta, with its twenty-six described orders. Insects first appear in the fossil record about 400 million years ago. Insects have three distinct segments (head, thorax, and abdomen). Two pairs of wings and three pairs of legs are typical. Scientists who study insects are called entomologists. Class Insecta is the largest class in the phylum Arthropoda, and their diversity is unmatched in the animal kingdom. Flight evolved independently in this group (what other types of animals can fly?) and is thought to be one factor in the dramatic success of the insects. Why would flight be correlated with species diversity?
Figure 29. Several respresentative hexapods. A. A cricket B. A solitary bee C. A leafhopper D. A butterfly. (Click image to enlarge)
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.
Arthropods fall into one of five subphyla. Segmentation, and subsequent specialization, is one of the hallmarks of each of these five groups. Trilobites were a very successful group that became extinct about 250 million years ago. Although these animals were segmented, they had very little diversification in their segments.
The chelicerates include spiders. These animals have two major body segments: the cephalothorax and the abdomen. Their appendages are all clustered in the cephalothorax. Typically there are six pairs of appendages. These include the chelicerae, which are involved in feeding, and the pedipalps, which may have a feeding function. The remaining appendages are walking legs. The members of this group do not have antennae but do have simple eyes.
Crustaceans, the second largest extant subgroup, include mostly aquatic species, however, there are some terrestrial species. They have two pairs of sensory antennae, jaw-like mandibles, and compound eyes.
The millipedes and centipedes are myriapods. They have many unspecialized segments, and many pairs of legs. The usually have one pair of antennae and simple eyes.
The hexapods are the largest extant subgroup. They have jaw-like mandibles, compound eyes, one pair of sensory antennae, and unbranched (uniramous) appendages. This subgroup includes Class Insecta, which have distinct segments that may be highly specialized. Flight evolved in this group, and is in part responsible for its success and diversity.
After reading this tutorial, you should have a working knowledge of the following terms:
Questions? Send your instructor a message through ANGEL!