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Introduction to Invertebrates
Introduction to Invertebrates
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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 Animals

Lesson Objectives

  • List the characteristics that define the animal kingdom.
  • Define and give examples of the invertebrates.

Check Your Understanding

  • What are the main differences between an animal cell and a plant cell?
  • How do animals get their energy?

Introduction

How 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 Animals

Recall 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.

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.

Phylum Examples
Porifera Sponges
Cnidaria Jellyfish, corals
Platyhelminthes Flatworms, tapeworms
Nematoda Nematodes, heartworm
Mollusca Snails, clams
Annelida Earthworms, leeches
Arthropoda Insects, crabs
Echinodermata Sea stars, sea urchins

Lesson Summary

  • Animals are multicellular, eukaryotic heterotrophs.
  • Animals can be classified by both molecular data and physical characteristics such as symmetry.
  • Invertebrates are animals without a backbone.

Review Questions

  1. What are some key features that define the animal kingdom?
  2. What does heterotrophic mean?
  3. What defines the invertebrates?
  4. What are some examples of invertebrates?
  5. What is the difference between radially and bilaterally symmetrical animals?
  6. What’s an example of a bilaterally symmetrical animal?
  7. What are some examples of a radially symmetrical animal?
  8. What is a body cavity?
  9. What is the difference between an incomplete and complete digestive system?
  10. What is segmentation?

Further Reading

Vocabulary

bilaterally symmetrical
Body plan in which the left and right side are mirror images.
complete digestive tract
A digestive tract that has two openings, the mouth and the anus.
heterotroph
Organism that cannot make its own food, so it must ingest some type of organic matter.
invertebrates
Animals without a backbone.
incomplete digestive tract
A digestive tract that has only one opening.
radially symmetrical
A body plan in which any cut through the center results in two identical halves.
segmentation
Repetition of body parts or segments.

Points to Consider

  • What do you think that jellyfishes and corals have in common?
  • Think of some examples of animals that are bilaterally symmetrical, where the left side is a mirror image of the right?

Sponges and Cnidarians

Lesson Objectives

  • Describe the key features of the Sponges.
  • Describe the key features of the Cnidarians.
  • List examples of the Cnidarians.

Check Your Understanding

  • How are animals classified?
  • What is an invertebrate?

Introduction

The 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.

Sponges

Sponges 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.

Cnidarians

The 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 Colonies

Some 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.

Corals are colonial cnidarians.

Lesson Summary

  • Sponges are sessile filter feeders without true tissues.
  • The cnidarians, such as jellyfish, are radially symmetrical with true tissues.
  • Colonial cnidarians include the Portuguese man-of-war and corals.

Review Questions

  1. What is the only animal to lack true tissues?
  2. In what phylum are the sponges?
  3. How do sponges gain nutrition?
  4. Cnidarians are radially symmetrical. What does this mean?
  5. What are some examples of cnidarians?
  6. How do cnidarians sting their prey?
  7. Describe the nervous system of the cnidarians.
  8. How is a jellyfish different from a Portuguese man-o-war?
  9. How are coral reefs built?
  10. Where are most cnidarians found?

Further Reading / Supplemental Links

Vocabulary

corals
Cnidarians that live on ocean reefs in colonies.
cnidarians
Invertebrates that have radial symmetry and include the jellyfish.
filter feeders
An organism that feeds by filtering organic matter out of water.
gastrovascular cavity
A large cavity having both digestive and circulatory functions.
medusa
Cnidarian with a bell-shaped body directed downward.
nematocysts
Specialized cells in cnidarians that can release a small thread-like structure and toxins to capture prey.
porifera
Filter-feeders with saclike bodies; known as the sponges.
polyp
Cnidarian with a cup-shaped body directed upward.
sessile
Permanently attached and not freely moving.

Points to Consider

  • How do you think that worms are different from sponges and cnidarians?
  • How do you think that worms might be similar to sponges and cnidarians?

Worms

Lesson Objectives

  • Describe the major features of the flatworms.
  • Describe the major features of the roundworms.
  • Describe the major features of the segmented worms.

Check Your Understanding

  • In terms of body structure, what does segmentation refer to?
  • What is a body cavity?

Introduction

Calling 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.

Flatworms

Worms 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.

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.

Roundworms

The 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.

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 Worms

The 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.

Earthworms are segmented worms.

Leeches are parasitic 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.

Type of Worm Body Cavity Segmented Digestive System Example
Flatworm No No Incomplete Tapeworm
Roundworm Yes No Complete Heartworm
Segmented Yes Yes Complete Earthworm

(Source: Jessica Harwood, License: CC-BY-SA)

Lesson Summary

  • The flatworms have no true body cavity and include free-living Dugesia and parasitic tapeworms.
  • The roundworms, which can also be parasitic or free-living, are non-segmented worms with a complete digestive tract and a primitive brain.
  • The segmented worms include the common earthworm and leeches.

Review Questions

  1. Are all worms classified into a single phylum?
  2. Describe the respiratory system of the flatworms.
  3. What is cephalization?
  4. Name a parasitic flatworm.
  5. How does the body plan of the roundworms differ from that of the flatworms?
  6. Describe the digestive system of roundworms.
  7. What features distinguish Phylum Annelida from the other worms?
  8. Describe the skeletal system of a segmented worm.
  9. Name a parasitic segmented worm.
  10. Earthworms are in what phylum?

Further Reading / Supplemental Links

Vocabulary

annelida
Invertebrate worms that have segmented bodies, such as earthworms.
cephalization
Having a head region with a concentration of sensory organs and central nervous system.
complete digestive tract
A digestive tract with two openings, a mouth and anus.
gastrovascular cavity
A large cavity having both digestive and circulatory functions.
hydroskeleton
Fluid-filled body cavity that provides support for muscle contraction.
incomplete digestive system
A digestive tract with only one opening.
nematoda
Invertebrate worms that include the roundworms.
platyhelmenthes
Invertebrate worms that include the flatworms and tapeworms.
segmentation
A body plan that has repeated units or segments.
tapeworms
Intestinal parasites in the phylum Platyhelmenthes.

Points to Consider

  • How might the vertebrates be different from the invertebrates?
  • Can you think of some examples of animals with a backbone?

Lab

Survey of Some Invertebrates

In 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:

  • compound and dissecting microscopes
  • slides and cover slips
  • pipettes
  • watch glass
  • culture of living hydra
  • Dugesia
  • construction paper
  • preserved earthworms
  • dissection kits

Procedure:

  1. Hydra
    1. With a pipette, pull up some of the material from the bottom of the culture dish. Then squeeze a coupe drops onto a clean slide and cover with a cover slip. Observe your hydra under the microscope and sketch one below.
  2. Dugesia
    1. With a pipette, place a couple Dugesia on a clean watch glass. Observe under the dissecting microscope. Sketch below, labeling the eyespots, auricles, and gastrovascular cavity.
    2. With a dark piece of paper, cover half the watch glass. Do the Dugesia seem to prefer the shade or the light? Movement in response to light is called phytotaxis.
  3. Earthworm
    1. Find the clitellum. What is its function?
    2. Touch the ventral side of the worm to feel the setae. What are their function?
    3. Lay the worm on the dissecting tray with the dorsal side up. Using the forceps and the scissors, carefully cut open the worm along a straight line from the clitellum to the mouth. Make sure to just cut the skin so you do not damage the internal organs. Sketch your worm below and label the following: aortic arches, crop, gizzard, pharynx, dorsal blood vessel, intestine, ventral nerve cord, and seminal vesicles.

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