Introduction and Goals
The previous tutorial covered the pseudocoelomates and introduced you to the mollusks and annelids, coelomates collectively known as the protostomes (Fig. 1). This tutorial will complete our analysis of the protostome coelomates by examining the most diverse branch of animals, the arthropods. The second half of this tutorial will introduce the last major lineage, the deuterostomes.
As you progress through this tutorial, try to distinguish between the arthropods and other animals classified under the bilateria. How are these groups of animals similar? What morphological and developmental patterns do they have in common? How do they differ? Once you complete this tutorial you should be able to:
- Explain why the animals in Phylum Arthropoda are thought to be so successful
- Name and discuss the major classes and subgroups of Phylum Arthropoda
- Describe the embryonic characteristics of deuterostomes
- Describe the major characteristics of Phylum Echinodermata
- Explain the characteristics of arthropods that have made them successful
- Review the diversity of arthropod groups, including trends in arthropod evolution
- Describe the characteristics of the echinoderms and discuss why they are members of the bilateria
Figure 1. Animal Diversity and Body Plans. (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. 2). Some researchers estimate that the number of species may exceed 10 million.
Figure 2. 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).
The arthropod body is covered by a cuticle (structurally different from the cuticle of plants but there are some functional similarities), composed primarily of the polysaccharide chitin (the same type of polymer used by fungi, as was mentioned in Tutorial 16, but with a harder character; chitin was also described in Tutorial2, 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 their 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. The only cumbersome feature 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 3. 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 (often grouped together with the head as the cephalothorax), and the abdomen. The rhinoceros beetle (Fig. 4) has a head and thorax that are fused (a cephalothorax) and a large abdomen. 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 4. 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. 5) are specialized through adaptation to different and diverse environments.
Figure 5. A Candy Cane Shrimp. An arthropod that is a crustacean. (Click image to enlarge)
Figure 6 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 6. 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 grous presented here may change as future molecular data provide a clearer insight into arthropod relationships.
Figure 7. 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 (Fig. 8) 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. The pedipalps have either a feeding or sensory function. Chelicerates lack antennae and have simple eyes.
Figure 8. 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. 9) 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. 10).
Figure 9. A Fiddler Crab. An arthropod that is a crustacean. (Click image to enlarge)
Figure 10. A millipede. An arthropod that is a myriapod. (Click image to enlarge)
The hexapoda, 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. 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 11. Several respresentative hexapods. A. A cricket B. A solitary bee C. A leafhopper D. A butterfly. (Click image to enlarge)
The Other Coelomates: Deuterostomes
To understand the evolutionary history of animals, scientists rely on several types of data including, but not limited to, the following: the overall morphology of the organism, DNA sequence comparisons, other similarities at the molecular level, and the developmental sequence of events in the embryo.
The sequence of events during embryonic development is the main characteristic used to distinguish protostomes and deuterostomes. Review Figure 12, which depicts the major developmental characters that distinguish a protostome from a deuterostome. Recall that, in general, in protostomes the mouth forms first, whereas in deuterostomes it forms secondarily (the two names are based upon this difference). Another important distinction is how the coelom forms; in protostomes it forms from a splitting of the mesoderm (schizocoelous), whereas in most deuterostomes the coelom forms from mesodermal outpocketings of the archenteron (enterocoelous).
In this and the next tutorial we will focus on the two major deutherostome phyla, Echinodermata and Chordata.
Figure 12. Protostome versus Deuterostome Development. (Click image to enlarge)
Representatives of the phylum Echinodermata are common inhabitants of coastal tide pools (you can see them in the saltwater aquarium at the HUB). Sea lilies, sea stars (starfish), sea cucumbers, and most other echinoderms are sessile or slow-moving animals. Some representatives are instantly recognized by their five-fold symmetry (having rays and/or arms in fives or multiples of five). Even though the radial symmetry of these adult animals would lead one to question their placement in the bilateria, the bilateral symmetry of the larval stage of these animals indicates that they are members of the bilateria lineage. Furthermore, these animals are coelomates since they have a large, fluid-filled cavity lined with mesoderm and usually a complete gut. Many species of sea stars have the ability to turn their stomachs inside out by everting them through their mouths. This allows them to initiate the digestion of their prey before introducing it to their body cavities.
A thin skin overlaying a hard, yet flexible, endoskeleton composed of calcium carbonate plates and spines characterizes the exterior of echinoderms. These plates have a very complex arrangement. Electron microscopy of the surface of a sea urchin demonstrates this elegant arrangement, a sponge-like mesh which creates a plate that allows for special structures to protrude through the endoskeleton for locomotion, feeding, gas exchange, and protection. These protrusions, typically described as tube feet (sucker-like appendages), enable echinoderms to move and provide them with the ability to grip and manipulate objects or prey.
Echinoderms also have a unique water vascular system (a network of hydraulic canals extending in from the tube feet and around the gut of the organism). This expansive network uses hydraulic pressure to manipulate the extension of the tube feet as the animal breathes, feeds, or moves across the ocean floor.
Figure 13. A Starfish. (Click image to enlarge)
Phylum Echinodermata: Classification
Members of the phylum Echinodermata are diverse. In the adult state it is often unclear how these animals are related. However, a close inspection of their anatomy and embryology reveals their common evolutionary history. Shown below are members from four echinoderm classes (you will not be tested on these classes).
Figure 14. A Sea Star, a member of the Class Asteroidea. (Click image to enlarge)
Figure 15. A Sea Urchin, a member of the Class Echinoidea. (Click image to enlarge)
Figure 16. A Sea Cucumber, a member of the Class Holothuroidea. (Click image to enlarge)
Figure 17. A Brittle Star, a member of the Class Ophiuroidea. (Click image to enlarge)
What Do Humans Have In Common With Sea Urchins?
The relationships between humans and sea urchins may not be obvious, however, common embryonic features reveal their common ancestry. As you have learned, stages of embryonic development have an important place in the classification of animal phyla. Whether it is the fate of the blastopore, the number of germ layers (diploblastic versus triploblastic), or the formation and origin of the tissue lining the body cavity, embryonic origins provide information about the relationships among members of the Kingdom Animalia.
The close phylogenetic relationship of humans (and other chordates) and echinoderms is also supported by DNA sequence data. As more data are collected, these relationships will be refined, but to date the evidence showing embryonic similarities between the species of these groups is compelling. Some scientists have even discussed placing Phylum Echinodermata closer to Phylum Chordata, or even within Phylum Chordata, due to the discovery of some early echinoderms that might have possessed pharyngeal slits and a tail (diagnostic chordate features that are discussed in the next tutorial). Furthermore, undiscovered species may also provide information on the relatedness of chordates and echinoderms. From what you should have learned, would a modern representative of these groups shed more light on this overall question, or could just as much be learned from the fossil of an adult animal? Why or why not?
This tutorial continued our discussion of the bilateria branch of the kingdom Animalia. When you think about bilateral symmetry, keep in mind that it is seen in animals that actively move through the environment. Bilaterally symmetrical animals not only have a single plane of symmetry, but their sensory and cephalic areas are found at or near the anterior of the animal, a characteristic called cephalization.
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 balance of the appendages is legs. This group does not have antennae but does 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.
The deuterostomes include the echinoderms and the chordates. The echinoderms are an exclusively marine group that show five-fold radial symmetry as adult, but are bilaterally symmetrical as larvae. In the next (and last) tutorial on animals, you will learn more about the chordates.
After reading this tutorial, you should have a working knowledge of the following terms:
Case Study for Animals IV
The Asian Longhorned Beetle (Anoplophora glabripennis) is a recent invasive species in North America and infestations have been found throughout the US and Canada. The source of this invasion is populations in China. The ALB lays it eggs under tree bark and its larvae tunnel within a living tree, disrupting the vascular tissue and weakening the tree; it can live in and kill many species of hardwood trees, including maple, elm, horse chestnut, ash, birch, poplar, and willow. Adult beetles feed on leaves, bark and shoots, causing further damage. Ecologists fear that millions of acres of forest could be destroyed if this invasive species becomes established in North America. Geneticists are studying the genome of this species to better understand the amount of genetic variation in an attempt to understand the invasive potential of this species.
Researchers at Penn State, led by Dr. Kelli Hoover, have a quarantined colony of these beetles and are exploring their potential for use in producing biofuels. Microorganisms living within the gut of these beetles (at least 24 genera of bacteria have been found) can break down lignin; ethanol can be formed by these microorganisms. This research team is sequencing the genomes of these microbes in order to isolate enzymes that are involved in this process.
- A recent report (Carter et al, 2010) determined that the North American populations have less genetic variation than the source populations in China. They made this determination, in part, by analyzing mitochondrial DNA. Using what you know about mitochondrial DNA, explain why this type of DNA is valuable for genetic studies that involve comparing a population to a parent (or source) population.
- What is the monomer of lignin that can be used to produce ethanol?
- What type of relationship exists between the beetles and the microorganisms in their gut?
- Describe how the activity of the larvae kills the tree in which they are living.
Carter M, Smith M, Harris, R. (2010) Genetic analyses of the Asian longhorned beetle (Coleoptera,
Cerambycidae, Anoplophora glabripennis), in North America, Europe and Asia. Biological Invasions, 12:1165--1182
Figure 18. A sign to help prevent the spread of the Asian Long-horned Beetle. (Click to enlarge)
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).