Chapter 23: Animals: The Invertebrates

 

  • A. Humans have characteristics that can be traced by millions of years to the invertebrates.

    B. Invertebrate animals are not primitive and evolutionarily stunted, but rather display adaptations to an amazing variety of environments.

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    I. Overview of the Animal Kingdom (Animalia)

  • A. General Characteristics
  • 1. Most animals are multicellular, heterotrophic, aerobic, reproduce sexually, and are motile at some point in their life cycle.

    2. Animal life cycles include a period of embryonic development; germ tissue layers&endash;the ectoderm, endoderm, and in most species, mesoderm&endash;give rise to adult organs.

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    B. Diversity in Body Plans

  • 1. Animals with backbones are vertebrates; those without a backbone are invertebrates.

    2. Body Symmetry and Cephalization

  • a. Animals show either radial (round) or bilateral (left and right sides) symmetry; bilateral animals also show anterior (head end), posterior (tail end), dorsal (back), and ventral (belly) orientations.

    b. Cephalization means having a definite head end, usually with feeding and sensory features.

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    3. Type of Gut

  • a. Some are saclike with one opening&endash;a mouth for food entry and waste exit.

    b. "Complete" digestive tracts have two openings (mouth and anus) for continuous food processing, often through specialized regions.

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    4. Body Cavities

  • a. A coelom (lined with peritoneum) is a space between the gut and body wall that allows internal organs to expand and operate freely.

    b. Some animals (flatworms) do not have a coelom but are packed solidly with tissue.

    c. Others, such as roundworms, have a "false" coelom (a pseudocoelum), not lined with peritoneum.

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    5. Segmentation

  • a. A segmented animal is composed of repeating body units&endash;for example, the earthworm.

    b. The segments may be grouped and modified for specialized tasks, as in insects.

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    • II. Origins

  • A. Where did the first animals come from?
  • 1. One hypothesis is that there was a compartmentalization of a ciliate like Paramecium.

    2. Another proposes that multicelled animals arose from colonial organisms like Volvox (a type of colonial green algae).

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    B. Perhaps the earliest animals resembled the present-day placozoan called Trichoplax.

  • 1. Two layers of cell make up its flattened body that displays no symmetry, no tissues, and no mouth (like pita bread!).

    2. Reproductive modes are as yet unknown for Trichoplax.

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    • III. Sponges&endash;Success in Simplicity

  • A. Sponges have an asymmetric body with NO true tissues, NO organs.
  • 1. Flattened cells cover the exterior.

    2. Collar cells line the interior chambers.

  • a. These move large volumes of water through body pores by their beating flagella.

    b. They also trap suspended food particles in their microvilli collars.

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    3. Between the two layers of cells there is a semifluid matrix with needlelike structures for support.

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    B. Sponges reproduce sexually (a free-living larva precedes the adult) and asexually by fragmentation or gemmules.

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    IV. Cnidarians (the jellies) &endash;Tissues Emerge

  • A. Cnidarians are tentacled, radial animals.
  • 1. This group includes jellyfishes, sea anemones, and hydrozoans (Hydra).

    2. The phylum name comes from their ability to sting by discharging nematocysts.

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    B. Cnidarian Body Plans

  • 1. The medusa resembles an umbrella and floats like a tentacle-fringed bell in the water.

    2. The polyp is tubelike and is usually attached to some substrate; it may be solitary or part of a colony.

    3. The digestive cavity is saclike (only a mouth) and can accommodate prey larger than the cnidarian itself.

    4. True tissues include an outer epidermis and an inner gastrodermis (epithelial layers with a jellylike mesoglea between.

    5. A nerve net that coordinates sensory and motor activities,

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    C. Stages in Cnidarian Life Cycles

  • 1. Reproduction may be sexual, with a planula larva stage, or it may be asexual.

    2. Gonads may be in the epidermis or gastrodermis.

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    • V. Acoelomate Animals&endash;and the Simplest Organ Systems

  • A. Organs and organ systems are present.

    B. Flatworms -- Phylum Platyhelminthes

  • 1. Common features include: saclike gut (but none in tapeworms), bilateral symmetry, cephalization, no coelom, and hermaphroditism (both sexes in one body).

    2. Turbellarians (planarians) possess a pharynx tube extends to feed on whole small animals or suck tissues from dead or wounded prey; they have protonephridia, with flame cells, to regulate body fluid volume and composition; asexual reproduction is by fission of the body.

    3. Flukes are internal parasites that require a primary host (such as a human) for sexual reproduction and an intermediate host (such as a snail) for development.

    4. Tapeworms are intestinal parasites of vertebrates, where they absorb predigested nutrients (they have no digestive tract); the body consists of an anterior scolex solely for attachment to the host’s gut and a string of proglottids, each of which possesses both male and female organs.

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    • VI. Roundworms -- Phylum Nematoda

  • A. Roundworms are pseudocoelomate worms that thrive in nearly every habitat on earth.

    B. Most are small and free living but some are parasitic on plants and animals&endash;for example, hookworms in the human intestine.

    C. They are bilateral, possess a slender tapered body with complete digestive tract in a pseudocoelom filled with fluid; a tough cuticle covers and protects the body.

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    VII. Focus on Health: A Rogue’s Gallery of Worms

    VIII. Two Major Divergences

  • A. Coelomate animals belong to two main groups:
  • 1. Protostomes: mollusks, annelids, and arthropods.

    2. Deuterostomes: echinoderms and chordates.

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    B. In protostomes the early embryonic cell divisions are "angled" (spiral cleavage) and the blastopore becomes the mouth.

    C. In deuterostomes, the zygote divides symmetrically (radial cleavage) and the blastopore becomes the anus.

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    IX. A Sampling of Mollusks -- Phylum Mollusca

  • A. Molluscan Diversity
  • 1. Features in common usually include a head, foot, shell, mantle, gills, and a radula.

    2. Gastropods include snails and slugs.

  • a. Literal meaning of "belly foot" describes the position of the digestive tract within the large muscular foot.

    b. Most of the other organs are located in the spiraled shell.

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    3. Bivalves include clams, scallops, and oysters.

  • a. The shell of two parts encloses the body.

    b. The mother-of-pearl lining of the shell may generate pearls.

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    4. Cephalopods include squids, octopuses, and nautiluses.

  • a. This group includes the largest invertebrates known; the giant squid can attain a length of over sixty feet.

    b. They are also the most intelligent of invertebrates.

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    • B. Evolutionary Experiments With Body Plans

  • 1. Embryonic torsion in gastropods places several organs toward the head end of the body.

    2. In bivalves there is no head but a large foot specialized for burrowing comprises the bulk of the body; water and suspended food are drawn in through siphons by the action of the cilia on the gills.

    3. The cephalopod body is modified for a highly active predatory life-style and includes tentacles, beaklike jaws, and jet propulsion by mantle contractions.

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    • X. Annelids -- Phylum Annelida -- Segmented!

  • A. There are three groups of annelid worms:
  • 1. Earthworms bear few setae; their habit of ingesting dirt particles while scavenging for organic matter makes them valuable tillers of the soil.

    2. Leeches are aquatic or semiaquatic predators of invertebrates or parasites of vertebrates.

    3. Polychaetes include wandering or tube-dwelling marine worms with tentacles and numerous setae.

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    B. Advantages of Segmentation

  • 1. Different parts can evolve separately to become specialized for different tasks.

    2. Leeches have suckers at both ends of the body.

    3. Polychaetes have fleshy appendages called parapods.

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    C. Annelid Adaptations&endash;A Case Study [Earthworm]

  • 1. Their segmentation is extensive and obvious; internal partitions define individual coelomic chambers, which are filled with fluid to provide a hydrostatic skeleton.

    2. Paired nephridia occur in nearly every segment.

    3. The digestive system is complete; the circulation closed.

    4. Two adjoining nerve cords extend from anterior to posterior.

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    • XI. Arthropods -- Phylum Arthropoda -- The Most Successful Organisms on Earth!

  • A. Arthropod Diversity
  • 1. Success is measured as having the most species, producing the greatest numbers of offspring, occupying the most habitats, possessing terrific defenses, and being able to exploit new resources.

    2. The four main lineages are trilobites (now extinct), chelicerates, crustaceans, and uniramians.

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    B. Adaptations of Insects and Other Arthropods

  • 1. Hardened Exoskeletons
  • a. It is a combination of protein and chitin (plus calcium in some) that is flexible, lightweight, yet protective.

    b. It is a barrier to water loss and can support a body deprived of water’s buoyancy.

    c. Exoskeletons restrict growth and so must be shed periodically (molting process).

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    2. Jointed Appendages

  • a. Body segments became reduced in number and more specialized.

    b. Appendages also became specialized for feeding, sensing, and locomotion.

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    3. Fused and Modified Segments

  • a. Body segments became more specialized, fewer, and grouped or fused.

    b. Different segments combined to form the head, thorax, and abdomen.

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    4. Respiratory Structures

  • a. Special tubes called tracheas supply oxygen directly to body tissues.

    b. This allows high metabolic rates and sustained activity, as in flight.

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    5. Specialized Sensory Structures

  • a. The compound eye provides a wide angle of vision.

    b. Many individual units allow motion perception.

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    6. Division of Labor

  • a. Metamorphosis transforms insects from immature larval stages through a pupal stage to adult.

    b. Larval stages concentrate on feeding and growth, whereas the adults specialize in dispersal and reproduction.

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    • XII. A Look At Spiders (Arachnids, in Arthropoda) and Their Kin

  • A. Chelicerates include the terrestrial spiders, scorpions, ticks, mites and the aquatic horseshoe crabs.
  • 1. Spiders are eight-legged predators that trap insects in their webs.

    2. Some mites are free living, others are serious pests of plants and animals; ticks are notorious blood-suckers and disease carriers.

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    B. Arachnid body features include chelicerae (piercing), pedipalps (grasping), open circulation, and book lungs (respiration).

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    XIII. A Look at the Crustaceans (also in Arthropoda)

  • A. Crustaceans include shrimps, lobsters, crayfishes, crabs, barnacles, and pillbugs.
  • 1. The name is derived from the crusty exoskeleton.

    2. Most are important components of food webs and several serve as human food also.

    3. Appendages on the body include two pairs of antennae, a pair each of mandibles and maxillae, and five pairs of legs.

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    B. Unusual crustaceans include the tiny copepods that are integral to marine food webs and the shell-encased barnacles that cause problems when they attach to wharf pilings.

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    XIV. How Many Legs? -- More Arthropods!

  • A. Millipedes and centipedes have a long, segmented body with many legs

    B. Millipedes are slow-moving vegetarians.

  • 1. The body is cylindrical.

    2. There two pairs of legs on each body segment.

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    C. Centipedes move rapidly and prey on small invertebrates mostly.

  • 1. The body is flattened.

    2. There is only one pair of legs per body segment.

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    • XV. A Look at Insect Diversity: The Class Insecta is also in the Phylum Arthropoda

  • A. Insects have several unique features:
  • 1. The body is divided into three regions: head (sensory and feeding), thorax (locomotion: by six legs, two pairs of wings), and abdomen.

    2. Unique Malpighian tubules process metabolic waste and aid in water retention.

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    B. Insects are enormously successful.

  • 1. They can produce enormous numbers of offspring.

    2. Their ability to disperse by flight allows the use of widely ranging food sources.

    3. Metamorphosis through larval, pupal, and adult stages allows fuller exploitation of nature resources.

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    • XVI. The Puzzling Echinoderms -- Phylum Echinodermata

  • A. These are the spiny skinned animals: sea stars, sea urchins, brittle stars, and sea cucumbers.
  • 1. Adults are radially symmetrical; larvae are bilateral.

    2. The decentralized nervous system permits response to be made in all directions.

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    B. The unique water vascular system operates the tube feet by contracting the ampulla on each one.

  • 1. Collectively the tube feet achieve a suction useful in locomotion and prey capture.

    2. Sea stars can evert their stomachs when feeding.

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