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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

Worms and Mollusks

Chapter 27



Section 27-1

What is a flatworm

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

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 flatworms1

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

Excretory System

Form and function in flatworms2

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 flatworms3

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 flatworms4

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

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

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 roundworm1

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

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 roundworms1

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 roundworms2

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

Roundworms and Human Disease

  • Parasitic roundworms include:

    • Trichinosis-causing worms

    • Filarial worms

    • Ascarid worms

    • Hookworms

Trichinosis causing worms

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

Filarial Worms

  • Threadlike worms that live in blood of birds and mammals

  • Causes elephantiasis

Ascarid worms

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

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

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 annelids1

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 annelids2

Form and Function in Annelids


Form and function in annelids3

Form and Function in Annelids

  • Response

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



Form and function in annelids4

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

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

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

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

Form and Function in Mollusks

  • True coeloms

  • Complex, interrelated organ systems

Form and function in mollusks1

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 mollusks2

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 mollusks3

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 mollusks4

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 mollusks5

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 mollusks6

Form and Function in Mollusks


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


Form and function in mollusks7

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 mollusks8

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

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

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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