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Introduction to Invertebrates
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Last modified: 1 month ago This chapter gives an overview of animals and further discusses sponges, cnidarians, and worms, including flatworms, roundworms, and segmented worms. 
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Introduction to Invertebrates
Overview of AnimalsLesson Objectives
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IntroductionHow are animals different from other forms of life? Recall that all animals are eukaryotic, meaning that they have cells with true nuclei and membrane-bound organelles. Another feature that distinguishes animals from animal-like protists is that animals are multicellular, while protists are often unicellular. Because animals are multicellular, animal cells can be organized into tissues, organs, and organ systems. Finally, animals are heterotrophic, meaning they must ingest some type of organic matter for nutrition and energy (Figure below). Eukaryotic, multicellular, and heterotrophic are features shared by all the millions of diverse types of animals on earth, from tiny ants and snails to giant whales and grizzly bears. In this chapter we will just focus on the invertebrates, the animals that do not have a backbone of bone or cartilage. Animals are heterotrophs, meaning they must eat to get molecules necessary for their growth and energy. Classification of AnimalsRecall that each kingdom of life, including the animal kingdom, is divided into smaller groups called phyla based on their shared characteristics. For example the phylum Mollusca largely consists of animals with shells like snails and clams. Although modern classification is also based on looking at molecular data, such as DNA sequencing, animals have long been classified in their current phyla largely by their physical characteristics. One example of a physical characteristic used to classify animals is body symmetry. In radially symmetrical organisms, such as sea stars, the body is organized like a circle (Figure below). Therefore, any cut through the center of the animal results in two identical halves. Other animals, such as humans and worms, are bilaterally symmetrical, meaning their left and right sides are mirror images. Sea stars are radially symmetrical. Animals are also often classified by their body structure. For example, segmentation, the repetition of body parts, defines one phylum of worms (Figure below). Animals that have a true body cavity, defined as a fluid-filled space, and internal organs are also classified in separate phyla from those animals that do not have a true body cavity. Finally, the structure of the digestive system of animals can also be used as a characteristic for classification. Animals with incomplete digestive tracts have only one opening in their digestive tracts, while animals with complete digestive tract have two openings, the mouth and anus. A segmented body plan defines the phylum that includes the earthworms. What Are Invertebrates?Besides being classified into phyla, animals are also often characterized as being invertebrates or vertebrates. This is an informal classification term based on the skeletons of the animals. Vertebrates have a backbone of bone or cartilage, while invertebrates have no backbone. All vertebrate organisms are in the phylum Chordata, while invertebrates make up several diverse phyla. As seen in Figure below, the invertebrates include the insects, the earthworms, the jellyfish, the star fish, and a variety of other animals. In the next lessons we will discuss some of phyla within the animal kingdom that contain invertebrates. Snails are an example of invertebrates, animals without a backbone.
Lesson Summary
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Further Reading
Vocabulary
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Sponges and CnidariansLesson Objectives
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IntroductionThe ocean is home to a variety of organisms. Phytoplankton, tiny photosynthetic organisms that float in the water, make their own food from the energy of the sun. Small aquatic animals, known as zooplankton, and larger animals, such as fish, use phytoplankton as a food source. These animals can in turn be eaten by larger aquatic animals, such as larger fish and sharks. Among the various types of animals that live in the ocean, the sponges and cnidarians are important invertebrates. The Sponges are believed to be one of the most ancient forms of animal life on earth. The cnidarians, which include the jellyfish, also are among the oldest and most unusual animals on earth. In this lesson we will discuss the features that make these two types of invertebrates unique from other types of animals. SpongesSponges are classified in the phylum Porifera, which derives its name from Latin words meaning “pore bearing.” These pores allow the movement of water into the sponges’ saclike bodies (Figure below). Sponges pump water through their bodies because they are sessile filter feeders, meaning they cannot move and must filter organic matter and tiny organisms out of the water to obtain food. Sponges have tube-like bodies with many pores. Sponges are relatively primitive animals and do not have brains, stomachs, or other organs. In fact, sponges do not even have true tissues. Instead, their bodies are made up of specialized cells that each has specific functions. For example, the collar cells are flagellated and encourage water movement, while other types of cells regulate the water flow by increasing or decreasing the size of the pores. CnidariansThe cnidarians, in the phylum Cnidaria, include organisms such as the jellyfish (Figure below) and sea anemones (Figures below and below) that are found in shallow ocean water. You might recognize that these animals can give you a painful sting if you step on them. That’s because cnidarians have stinging cells known as nematocysts. When touched, the nematocysts unleash long, hollow threads that are intended to trap prey, and sometimes toxins are also injected through these threads to paralyze the prey. The body plan of cnidarians is unique because these organisms are radially symmetrical, meaning that they have a circular body plan so that any cut through the center of the animal leaves two equal halves. The cnidarians have two basic body forms, polyp and medusa. The polyp is a cup-shaped body with the mouth directed upward, such as a sea anemone (Figure below). The medusa is a bell-shaped body with the mouth and tentacles directed downward, such as a jellyfish (Figure below). Unlike the sponges, the cnidarians are made up of true tissues. The inner tissue layer secretes digestive enzymes into the gastrovascular cavity, a large cavity that has both digestive and circulatory functions. The cnidarians also have nerve tissue organized into a net-like structure. Cnidarians do not have true organs, however. Jellyfish have bell-shaped bodies with tenticles. Sea anemones can sting and trap fish with their tentacles. One type of sea anemone is home to the clownfish. Cnidarian ColoniesSome types of cnidarians are also known to form colonies. For example, the Portuguese man-of-war looks like a single organism but is actually a colony of polyps (Figure below). One polyp is filled with air to help the colony float, while several feeding polyps hang below with tentacles full of nematocysts. Consequently, the Portuguese man-of-war is known to cause extremely painful stings to swimmers and surfers who accidentally brush up against these creatures in the water. The Portuguese man-o-war can deliver debilitating stings with its tentacles. Coral reefs are built from colonial cnidarians called corals (Figure below). The corals are sessile polyps that can extend their tentacles to feed on ocean creatures that pass by. Their skeletons are made up of calcium carbonate, which is also known as limestone. Over long periods of time, their skeletons can accumulate to produce massive structures known as coral reefs. Coral reefs are important habitats for diverse types of ocean life. Corals are colonial cnidarians. Lesson Summary
Review Questions
Further Reading / Supplemental Links
Vocabulary
Points to Consider
WormsLesson Objectives
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IntroductionCalling an animal a worm is an informal, non-scientific classification for animals that have long bodies with no appendages. Worms are bilaterally symmetrical, meaning that the right side of their bodies is a mirror of the left. Worms live in a variety of environments, including in the ocean, in fresh water, on land, and as parasites of plants and animals. In this chapter we will discuss three types of worms: the flatworms, the roundworms, and the segmented worms. These worms are distinguished from each other by their body plan. The flatworms have flat ribbon-like bodies with no body cavity. The roundworms have a body cavity but no segments. The segmented worms have both a body cavity and segmented bodies. FlatwormsWorms in the phylum Platyhelminthes are called flatworms because they have flattened bodies. Some species of flatworms are free live-living organisms that feed on small organisms and decaying matter. These types of flatworms include marine flatworms and fresh-water flatworms such as Dugesia (Figures below and below). Other types of flatworms are parasitic and rely on a host organism for energy. For example, tapeworms have a modified head region with tiny hooks that help the worm attach to the intestines of a animal host (Figures below and below). is a type of flatworm with a head region and eyespots. Marine flatworms can be brightly colored. Tapeworms are parasitic flatworms that live in the intestines of their hosts. Tapeworms attach to the intestinal wall with a head region that has hooks and suckers. Flatworms have no true body cavity and an incomplete digestive system, meaning that the digestive tract has only one opening. Flatworms do not have a respiratory system, so gas exchange occurs at surface of their bodies. Furthermore, there are no blood vessels or true circulatory system in the flatworms. Their gastrovascular cavity serves for both digestion and to distribute nutrients. The flatworms do have a ladder-like nervous system with a distinct head region with a concentration of nerve cells and sensory organs such as eyespots (Figure ref|LS11-0301|below}}). The development of a head region, called cephalization, arose with the development of bilateral symmetry in animals. RoundwormsThe phylum Nematoda includes non-segmented worms known as nematodes or roundworms (Figure below). Unlike the flatworms, the roundworms have a body cavity with internal organs. A roundworm’s complete digestive tract, meaning the digestive tract includes both a mouth and anus, includes a large digestive organ known as the gut. Roundworms also have a simple nervous system with a primitive brain. Both their anterior and posterior ends have specialized sensory nerves. These nerves are connected with a ventral and dorsal nerve cord that run the length of the body. Nematodes can be parasites of plants and animals. Roundworms can be free-living organisms, but they are probably best known for their role as significant plant and animal parasites. The heartworms, which cause serious disease in dogs while living in the heart and blood vessels, are a type of roundworm. Round worms can also cause disease in humans. Elephantiasis, a disease characterized by the extreme swelling of the limbs, is caused by infection with a type of roundworm (Figure below). One roundworm parasite causes elephantiasis, a disease characterized by the swelling of the limbs. Segmented WormsThe phylum Annelida includes the segmented worms such as the common earthworm, some marine worms, and leeches (Figures below and below). These worms are known as the segmented worms because their bodies are segmented, or separated into repeating units. Most segmented worms feed on dead organic matter, while leeches can live in freshwater and suck blood from host organisms. Leeches can also be used medicinally to remove excess blood. Earthworms are segmented worms. Leeches are parasitic segmented worms. Segmented worms have a well-developed body cavity filled with fluid, which serves as a hydroskeleton, a supportive structure that aids in muscle contraction. Segmented worms also tend to have organ systems that are more developed than the roundworms or flatworms. Earthworms, for example, have a complete digestive tract including an esophagus and intestines. The circulatory system consists of paired hearts and blood vessels, while the nervous system consists of the brain and a ventral nerve cord.
(Source: Jessica Harwood, License: CC-BY-SA) Lesson Summary
Review Questions
Further Reading / Supplemental Links
Vocabulary
Points to Consider
LabSurvey of Some InvertebratesIn this lab you will observe some examples of the invertebrates, those animals that do not have a backbone. The hydras are in the phylum Cnidaria. The Dugesia are in the phylum Platyhelmenthes, the flatworms. The earthworm is in the phylum Annelida. Materials:
Procedure:
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