Worms and mollusks
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Worms and Mollusks. Chapter 27. Flatworms. Section 27-1. What is a flatworm?. Phylum Platyhelminthes No more than a few millimeters thick Have tissues and internal organ systems Have bilateral symmetry and cephalization

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Worms and Mollusks

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Worms and Mollusks

Chapter 27


Section 27-1

What is a flatworm?

  • Phylum Platyhelminthes

  • No more than a few millimeters thick

  • Have tissues and internal organ systems

  • Have bilateral symmetry and cephalization

  • Known as acoelomates (“without coelem,” which is a fluid filled body cavity)

Form and Function in Flatworms

  • Feeding

  • Digestive cavity with a single opening through which both food and wastes pass

  • Parasitic worms obtain nutrients from foods that have already been digested by their hosts

Form and Function in Flatworms

  • Respiration, Circulation, and Excretion

  • Rely on diffusion to:

    • Transport oxygen and nutrients to internal tissues

    • Remove carbon dioxide and other wastes from their bodies

  • Have no gills or respiratory organs, heart, blood vessels, or blood

  • Some have flame cells – remove excess water and filter wastes from the body

Excretory System

Form and Function in Flatworms

  • Response

  • A head encloses several ganglia (groups of nerve cells) that control the nervous system

  • Have eyespots that look like eyes, but are groups of cells that can detect changes in the amount of light in their environment

Form and Function in Flatworms

  • Movement

  • Two means of movement:

    • Cilia to help glide through water

    • Muscle cells allow them to twist and turn

Form and Function in Flatworms

  • Reproduction

  • Most are hermaphrodites that reproduce sexually

    • A hermaphrodite is an individual that has both male and female reproductive organs

  • Asexual reproduction takes place by fission, in which an organism splits in two

Groups of Flatworms

  • Three main groups of Flatworms

    • Turbellarians

    • Flukes

    • Tapeworms

  • Most turbellarians are free-living

  • Most other flatworm species are parasites


  • Free-living flatworms

  • Live in marine or freshwater


  • Class Trematoda

  • Parasitic flatworms

  • Infect the internal organs of their host


  • Class Cestoda

  • Long, flat, parasitic worms

  • Adapted to life inside the intestines of their host

    • No digestive tract

    • Absorb already digested nutrients from host


Section 2

What is a Roundworm?

  • Phylum Nematoda

  • Slender, unsegmented worms with tapering ends

  • Range in size from microscopic to a meter

  • Most are free-living, inhabiting soil and water

  • Others are parasitic

What is a Roundworm?

  • Have a pseudocoelom (“false coelom”)

  • Have a digestive tract with two openings – a mouth and an anus

Form and Function in Roundworms

  • Have specialized tissues and organ systems

  • Body systems of free-living roundworms are more complex than parasitic ones

Form and Function in Roundworms

  • Feeding

    • Predators that use grasping mouthparts to catch and eat small animals

  • Respiration, Circulation, and Excretion

    • Exchange gases and excrete metabolic waste through their body walls

    • Depend on diffusion to carry nutrients and waste through their bodies

Form and Function in Roundworms

  • Response

    • Simple nervous systems

    • Have several types of sense organs

  • Movement

    • Muscles extend the length of their bodies

  • Reproduction

    • Reproduce sexually, and most have separate sexes

Roundworms and Human Disease

  • Parasitic roundworms include:

    • Trichinosis-causing worms

    • Filarial worms

    • Ascarid worms

    • Hookworms

Trichinosis-Causing Worms

  • Trichinosis – terrible disease caused by the roundworm Trichinella

  • Adult worms live and mate in the intestines of their hosts

  • Humans usually get the disease from eating undercooked pork

Filarial Worms

  • Threadlike worms that live in blood of birds and mammals

  • Causes elephantiasis

Ascarid Worms

  • Serious parasite of vertebrate animals

  • Causes malnutrition in more than 1 billion people worldwide

  • Absorbs digested food from the host’s small intestine


  • 25% of people in the world are affected with hookworms

  • Live in host’s intestines

  • Feed on blood, causing weakness and poor growth


Section 27-3

What is an Annelid?

  • Phylum Annelida

  • Worms with segmented bodies

    • Each segment is separated by a septum

  • Have a true coelom

Form and Function in Annelids

  • Feeding and Digestion

    • Many get their food using a pharynx

      • Food moves from the pharynx, into the esophagus, the crop, the gizzard, and then to the intestine

    • Others obtain food by filter feeding

  • Circulation

    • Closed circulatory system – blood is contained within a network of blood vessels

Form and Function in Annelids

  • Respiration

    • Aquatic annelids often breath through gills

    • Land-dwelling annelids take in oxygen and give off carbon dioxide through their moist skin

  • Excretion

    • Digestive wastes pass through the anus at the end of the digestive tract

    • Cellular waste is eliminated through nephridia (excretory organs)

Form and Function in Annelids


Form and Function in Annelids

  • Response

    • Well developed nervous system consisting of a brain and several nerve cords



Form and Function in Annelids

  • Movement

    • Two groups of muscles that work together as part of a hydrostatic skeleton

  • Reproduction

    • Most reproduce sexually

      • Some have separate sexes, others are hermaphrodites

Groups of Annelids

  • Three classes of Annelids

    • Oligochaetes

    • Leeches

    • Polychaetes


  • Class Oligochaeta

  • Contains earthworms and their relatives

  • Streamlined bodies

  • Relatively few setae

  • Most live in soil or freshwater


  • Class Hirudinea

  • External parasites that suck the blood and body fluids of their host


  • Class Polychaeta

  • Contains sandworms, blood worms, and relatives

  • Marine annelids that have paired, paddlelike appendages tipped with setae (brushlike structures)

Ecology of Annelids

  • Earthworms and many other annelids spend their lives burrowing through soil, aerating and mixing it

  • Earthworms help plant matter decompose

  • Earthworm castings are rich in nitrogen, phosphorus, potassium, micronutrients, and beneficial bacteria


Section 4

What is a Mollusk?

  • Soft-bodied animals

  • Usually have an internal or external shell

  • Free-swimming larval stage called a trocophore

Form and Function in Mollusks

  • True coeloms

  • Complex, interrelated organ systems

Form and Function in Mollusks

Body Plan

  • Variation on four main parts:

    • Foot – takes many forms

    • Mantle – layer of tissue that covers the

      mollusk’s body

    • Shell – made by glands in the mantle

    • Visceral mass – consists of internal organs

Form and Function in Mollusks


  • Can be herbivores, carnivores, filter feeders, detritivores, or parasites

  • Snails and slugs feed using a tongue-shaped structure called a radula

Form and Function in Mollusks


  • Clams, oysters, and scallops use gills

  • Food enters through a siphon – tubelike structure through which water enters and leaves the body

Excurrent siphon

Incurrent siphon

Form and Function in Mollusks


  • Aquatic mollusks breathe using gills inside their mantle cavity

  • Land snails and slugs respire through the moist surface of their skin


Form and Function in Mollusks


  • Some have open circulatory systems – works well for slow-moving mollusks (snails and clams)

  • Others have closed circulatory systems – works best for fast moving mollusks (octopi and squid)


Form and Function in Mollusks


  • Nephridia remove wastes from the blood and release it outside the body


Form and Function in Mollusks


  • Two-shelled mollusks have simple nervous systems

  • Octopi and relatives have the most highly developed nervous systems of all invertebrates

  • Octopus opening a jar

Form and Function in Mollusks


  • Move in a variety of ways

  • Snails secrete mucus and move over surfaces using the foot

  • Octopi use a form a jet propulsion


  • Reproduce in a variety of ways

  • Snails and two-shelled mollusks: external fertilization (sexually)

  • Tentacled mollusks and some snails: internal fertilization (sexually)

Groups of Mollusks

  • Three major classes:

    • Gastropods

    • Bivalves

    • Cephalopods


  • Class Gastropoda

  • Shell-less or single-shelled

  • Move using muscular foot on ventral side

  • Includes: pond snails, land slugs, sea butterflies, sea hares, limpets, and nudibranchs


  • Class Bivalvia

  • Have two shells held together by one or two powerful muscles

  • Include: clams, oysters, mussels, and scallops


  • Class Cephalopoda

  • Soft-bodied

  • Head is attached to a single foot

  • Foot is divided into tentacles or arms

  • Includes: octopi, squids, cuttlefishes, and nautiluses

Ecology of Mollusks

  • Mollusks play many different roles in living systems:

    • Feed on plants

    • Prey on animals

    • Filter algae out of the water

    • Eat detritus

  • Some mollusks are hosts to symbiotic algae or to parasites; others are themselves parasites

  • Mollusks are food for many organisms

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