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Worms and Mollusks. Biology 112. Flatworms. Phylum Platyhelminthes Soft, flattened worms with tissues and internal organ systems The simplest animals with three germ layers, bilateral symmetry and cephalization Acoelomates No coelom between the tissues of flatworms

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Worms and mollusks
Worms and Mollusks

Biology 112


  • Phylum Platyhelminthes

  • Soft, flattened worms with tissues and internal organ systems

  • The simplest animals with three germ layers, bilateral symmetry and cephalization

  • Acoelomates

    • No coelom between the tissues of flatworms

    • A coelom is a fluid-filled body cavity that is lined with tissue in the mesoderm

Body functions of flatworms
Body Functions of Flatworms

  • All use diffusion for distribution of nutrients and gases

  • Some have specialized flame cells used to remove excess water from the body

  • Contain a digestive cavity with one opening – a mouth

  • A muscular tube, called a pharnyx, pumps food into the digestive cavity

Classification of flatworms
Classification of Flatworms

  • Tubellarians

    • Free-living flatworms, can live in fresh and salt water

    • Ganglia control the nervous system

    • Some contain an eyespot to detect light

    • Reproduce asexually and sexually

    • Planarians are the most familiar species

Classification of flatworms1
Classification of Flatworms

2. Flukes

  • Parasitic

  • Infect the internal organs of their host

  • Reproduce sexually in the primary host and asexually in the intermediate host

Classification of flatworms2
Classification of Flatworms

3. Tapeworms

  • long, flat parasitic flatworms

  • Live in the intestines of their hosts

R oundworms

  • Phylum Nematoda

  • Unsegmented worms that have pseudocoeloms

  • A body cavity that lies between the endoderm and the mesoderm tissues

  • Also have a digestive system with a mouth and an anus

Roundworms and body functions
Roundworms and Body Functions

  • Rely on diffusion for respiration, circulation and excretion

  • Muscles and fluid in the pseudocoelom act as a hydrostatic skeleton

  • Reproduce sexually by internal fertilization

Characteristics of roundworms
Characteristics of Roundworms

  • Most are free-living but some are parasitic

  • Parasitic include:

    • Trichinosis-causing worms

    • Filarial worms

    • Ascarid worms

    • Hookworms


  • Phylum Annelida

  • Earthworms

  • Segmented bodies

  • Have a true coelom that is lined with tissue from the mesoderm

  • Internal walls called septa separate the segments that divide the body

  • Most segments are similar to one another

  • A few are specialized with eyes or antennae

  • Many have bristles called setae attached to each segment

Internal structure of annelids
Internal Structure of Annelids

  • Complex organ structures

  • Many contain a pharynx that is used to get their food

  • Food moves through the crop, where it is stored

  • Then moves to the gizzard, an organ that grinds it into smaller pieces

  • Have a closed circulatory system in which blood is contained in vessels


  • Class Oligochaeta

  • Hermaphroditic

    • Has both male and female reproductive organs

    • When eggs are ready to be fertilized, a clitellum, a band of thickened segments, secretes a mucous ring in which fertilization takes place

  • Streamlined body with few setae

  • Can live in fresh water or soil


  • Phylum Mollusca

  • Soft-bodied animals that often have an internal or external shell

  • Many have a larval stage called a trochophore

  • Can be herbivores, carnivores, detritophores or parasites

Body parts of a mollusk
Body Parts of a mollusk

  • Foot

    • Used for crawling, burrowing and capturing prey

  • Mantle

    • A thin layer of tissue that covers most of the body

  • Visceral mass

    • Makes up the internal organs

  • Shell

    • Glands secrete calcium carbonate

Body systems of the mollusk
Body systems of the mollusk

  • Either closed or open circulatory systems

  • When it is closed, blood is pumped through vessels into a large saclike structures called sinuses

  • When it is open, blood is contained in blood vessels throughout the body

  • Nervous systems varies from simple to complex

Classification of mollusks
Classification of mollusks

  • Gastropods

    • Includes pond snails and land slugs

    • Single-shelled or shell-less

    • Move by using a muscular foot on their left ventral side

    • Use a flexible tongue called a radula to eat

  • Bivalves

    • Clams, oysters, mussels and scallops

    • Two-shells held together by muscles

  • Cephalopods

    • Includes octopus and squid

    • Soft-bodied mollusks in which the head is attached to a single foot

    • Foot is divided into tentacles

    • Small internal shells or no shell at all

    • Have complex sense organs