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Invertebrate Classification Douglas Wilkin, Ph.D. Jean Brainard, Ph.D. Say Thanks to the Authors Click http://www.ck12.org/saythanks (No sign in required) To access a customizable version of this book, as well as other interactive content, visit www.ck12.org CK-12 Foundation is a non-profit organization with a mission to reduce the cost of textbook materials for the K-12 market both in the U.S. and worldwide. Using an open-content, web-based collaborative model termed the FlexBook®, CK-12 intends to pioneer the generation and distribution of high-quality educational content that will serve both as core text as well as provide an adaptive environment for learning, powered through the FlexBook Platform®. 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Printed: October 23, 2012 AUTHORS Douglas Wilkin, Ph.D. Jean Brainard, Ph.D. www.ck12.org C ONCEPT Concept 1. Invertebrate Classification 1 Invertebrate Classification • Give an overview of invertebrate classification. Invertebrate, arthropod or insect? This Monarch caterpillar is an invertebrate. It is also an insect and an arthropod. Of all the animal species, it is estimated that well over 90% are invertebrates. Of all invertebrates, the insects are by far the most numerous. There are so many species of insects that scientists have yet to discover them all, let alone name or count them. Estimates of the total number of insect species fall in the range of 1 to 30 million. So, it helps if there are methods to classify not just the insects, but all invertebrates. Classification of Invertebrates Eight major phyla contain the majority of invertebrate species. Major Invertebrate Phyla Table 1.1 gives an overview of the eight invertebrate phyla with the greatest number of species. TABLE 1.1: Major Invertebrate Phyla Phylum (includes) Porifera (sponges) Cnidaria (jellyfish, corals) Platyhelminthes (flatworms, tapeworms, flukes) Nematoda (roundworms) Mollusca (snails, clams, squids) Annelida (earthworms, leeches, marine worms) Arthropoda (insects, spiders, crustaceans, centipedes) Echinodermata (sea stars, sea urchins, sand dollars, sea cucumbers) Notable Characteristics multicellularity, specialized cells but no tissues, asymmetry, incomplete digestive system radial symmetry, true tissues, incomplete digestive system cephalization, bilateral symmetry, mesoderm, complete digestive system pseudocoelom, complete digestive system true coelom, organ systems, some with primitive brain segmented body, primitive brain Example sponges segmented body, jointed appendages, exoskeleton, brain complete digestive system, coelom, spiny internal skeleton insect (dragonfly) jellyfish flatworm roundworm snail earthworm sea urchin Protostomes and Deuterostomes Most invertebrates (and higher animals) can be placed in one of two groups based on how they develop as embryos. The two groups are called protostomes and deuterostomes. As shown in Figure 1.1, organisms in the two groups 1 www.ck12.org have different ways of forming the coelom and mouth, among other differences. Mollusks, annelids, and arthropods are protostomes. Echinoderms and chordates are deuterostomes. This distinction is important. Why does it matter? It shows that echinoderms are more closely related to chordates than are the other invertebrate phyla. This is not apparent based on other, more obvious traits. FIGURE 1.1 Protostomes vs. Deuterostomes. In protostomes such as mollusks, the coelom forms within the mesoderm. In deuterostomes such as echinoderms, the coelom forms from a pouch of endoderm. How does the formation of the mouth differ in these two groups of animals? Vocabulary • Annelida: Invertebrate phylum of segmented worms, such as earthworms. • Arthropoda: Invertebrate phylum of animals characterized by a segmented body, jointed appendages, exoskeleton, and brain. • Cnidaria: Invertebrate phylum that includes animals such as jellyfish and corals; characterized by radial symmetry, tissues, and a stinger called a nematocyst. • deuterostomes: Animals in which the coelom forms from a pouch of endoderm. • Echinodermata: Invertebrate phylum of sea-dwelling animals characterized by complete a digestive system, coelom, spiny internal skeleton. • Mollusca: Phylum of invertebrates that are generally characterized by a hard outer shell, a mantle, and a feeding organ called a radula. • Nematoda: Phylum of invertebrates called roundworms, which have a pseudocoelom and complete digestive system. • Platyhelminthes: Invertebrate phylum of flatworms; characterized by a flat body because they lack a coelom or pseudocoelom. • Porifera: Invertebrate phylum of sponges; have a non-bony endoskeleton and are sessile as adults. • protostomes: Animals in which the coelom forms within the mesoderm. 2 www.ck12.org Concept 1. Invertebrate Classification Summary • Eight invertebrate phyla contain most invertebrate species. • Invertebrates (and higher animals) can also be placed in one of two groups based on how they develop as embryos. Practice Use this resource to answer the questions that follow. • http://www.hippocampus.org/Biology → Biology for AP* → Search: Protostomes and Deuterostomes 1. 2. 3. 4. 5. What is the main difference between protostomes and deuterostomes? What is meant by "mouth first or second"? Compare the location of the mesoderm in protostomes and deuterostomes. Which develops first in annelids, the mouth or anus? Give three examples of annelids. Review 1. Describe evidence showing that echinoderms are more closely related to chordates than are other invertebrate phyla. 2. Assume you have discovered a new invertebrate. It has a segmented body, a brain, and jointed appendages. In which phylum would you place it? Why? 3