Worms and Mollusks

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Worms and Mollusks

• Biology I Chapter 27

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FLATWORMS
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Flatworms

• Phylum Platyhelminthes
• Soft, flattened worms
• Tissues and internal organs

• Simplest animals to have
• 3 embryonic germ layers
• Bilateral symmetry (a right and a left)
• Cephalization

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Flatworms

• Acoelomate flatworms are without a coelom
• Coelom fluid-filled body cavity lined with
  mesoderm
• The digestive cavity is the only body cavity

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Feeding

• Free-living flatworms
• Carnivores that feed on tiny aquatic animals
• Scavengers that feed on recently dead animals
• Parasitic flatworms
• Feed on blood, tissue fluids, or pieces of cells
  within a hosts body (Example tapeworm)

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Respiration, Circulation, Excretion

• Rely on diffusion
• Flame cells remove excess water and metabolic
  wastes from the body

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Response

• Ganglia group of nerve cells that controls the
  nervous system in the head region
• Eyespot

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Movement

• Cilia
• Muscle cells

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Reproduction Sexually and Asexually

• Free-living
• Sexually hermaphrodites
• during sexual reproduction, two worms join in a
  pair, delivering sperm to each other
• Asexually fission

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Reproduction Sexually and Asexually

• Parasitic
• A complex life cycle including both sexual and
  asexual reproduction

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Groups of Flatworms

• Turbellarians
• Free-living most live in marine or fresh water
• Flukes
• Parasitic infect the internal organs of their
  host
• Tapeworms
• Long, flat, parasitic
  adapted to life inside
  the
  intestines of their host

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ROUNDWORMS
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Roundworms

• Phylum Nematoda
• Slender, unsegmented worm
• Pseudocoelom
• Digestive system with two openings a mouth and
  an anus

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Feeding

• Free-living roundworms
• Carnivores that use grasping mouthparts and spines

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Respiration, Circulation, Excretion

• Exchange gases and excrete metabolic waste
  through their body walls
• No internal transport system
• Depend on diffusion

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Response

• Simple nervous systems, consisting of several
  ganglia
• Run from the head to the tail
• Nerves transmit sensory information and control
  movement

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Movement

• Hydrostatic skeleton
• Aquatic roundworms move like snakes
• Soil-dwelling roundworms push their way through
  by thrashing around

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Reproduction

• Sexually
• Separate males and females
• Internal fertilization
• Male deposits sperm inside the females
  reproductive tract
• Parasitic roundworms have complex life cycles
  involving two or three different hosts or organs
  within a single host

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Roundworms and Human Disease

• Trichinosis-causing worms
• Adult worms live and mate in the intestines of
  their host (humans, pigs and other mammals)
• Filarial worms
• Found primarily in tropical regions of Asia,
  threadlike worms that live in the blood and lymph
  vessels of birds and mammals, including humans,
  transmitted by biting insects, causes
  elephantiasis

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Roundworms and Human Disease

• Ascarid worms
• Serious parasite of humans and many other
  vertebrates, causes malnutrition spread by
  eating vegetables or food that are not washed
  properly
• Hookworms
• Hatch outside the body of the host and develop in
  the soil, can enter a barefoot and travel through
  the bloodstream to the intestines

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Research on C. elegans

• Free-living roundworm Caenorhabditis elegans, C.
  elegans
• Feeds on rotting vegetation
• First multicellular animal whose DNA was fully
  sequenced
• Helps understand genes and how eukaryotes became
  multicellular

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ANNELIDS
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Annelids

• True coelom that is lined
• Segmented bodies
• Septa internal walls between each segment
• Setae bristles that are attached to each
  segment used in respiration

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Form and Function in Annelids

• Have complex organ systems
• Segmented body

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Feeding and Digestion

• Filter feeders to predators
• Get their food using a pharynx
• Crop in earthworms part of the digestive system
  in which food can be stored
• Gizzard in earthworms part of the digestive
  system in which food is ground into smaller
  pieces

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Circulation

• Closed circulatory system blood is contained
  within a network of blood vessels
• Blood circulates through two major blood vessels
  that run from head to tail

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Respiration

• Aquatic annelids gills
• Land-dwelling annelids diffusion through their
  moist skin

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Excretion

• Two kinds of waste
• Digestive waste passes out through the anus at
  the end of the digestive tract
• Nephridia excretory organs that filter fluid in
  the coelom

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Response

• Well-developed nervous system consisting of a
  brain and several nerve cords
• The sense organs are best developed in
  free-living marine annelids

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Movement

• Hydrostatic skeleton
• Longitudinal muscles and circular muscles
• Moves by alternating contracting these two sets
  of muscles

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Reproduction

• Sexually
• Some use external fertilization or have separate
  sexes
• Others, such as earthworms and leeches, are
  hermaphrodites
• Exchange sperm
• Clitellum a band of thickened, specialized
  segments that secretes a mucus ring into which
  eggs and sperm are released and fertilization
  occurs
• The ring slips off the body and form a protective
  cocoon for the worms that hatch a week later

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Groups of Annelids

• Oligochaetes
• Streamlined bodies and have relatively few setae
  compared to polychaetes, live in soil or fresh
  water
• Leeches
• External parasites that suck the blood and body
  fluids of their host
• Polychaetes
• Marine annelids that have paired, paddlelike
  appendages tipped with setae

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Ecology of Annelids

• Provide passageways for plant roots and water and
  allow the growth of beneficial, oxygen requiring
  soil bacteria
• Important in the diet of many birds, moles,
  skunks, toads and snakes
• In the sea they participate in a wide range of
  food chains

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MOLLUSKS
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Mollusks

• Phylum Mollusca
• Soft-bodied animals
• Internal or external shell
• Include snails, slugs, clams, squids and octopi
• Trochophore free-swimming larval stage of an
  aquatic mollusk

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Form and Function in Mollusks

• True coeloms
• Have complex organ systems

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Body Plan

• Foot muscular part of a mollusk
• Mantle thin layer of tissue that covers most of
  a mollusks body

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Body Plan

• Shell structure in mollusks made by
  
  glands in the mantle that secrete calcium
  carbonate
• Visceral mass area beneath the mantle of a
  mollusk that contains the internal organs

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Feeding

• Herbivorous
• Carnivores
• Filter feeders
• Detritivores
• Parasites
• Radula flexible, tongue-shaped structure used to
  capture food by snails and slugs
• Siphon tube-like structure through which water
  enters and leaves the body, capturing plankton in
  the process

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Respiration

• Gills inside their mantle cavity
• Land snails respire using a mantle cavity lined
  with blood vessels
• Typically live in moist places to keep this
  lining wet

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Circulation

• Open circulatory system blood is pumped through
  vessels by a simple heart
• Works well for slow-moving mollusks such as
  snails and clams (demands for oxygen are low)
• Closed circulatory system can transport blood
  through an animals body much more quickly

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Excretion

• Cells of the body release nitrogen-containing
  waste into the blood in the form of ammonia
• Nephridia remove ammonia from the blood and
  release it out of the body

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Response

• Complexity of the nervous system varies greatly
  between mollusks
• Clams and other two-shelled mollusk lead inactive
  lives? simple nervous system

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Response

• Octopi and their relatives are active and
  intelligent predators ? most highly developed
  nervous system of all invertebrates
• Capable of complex behavior, such as opening a
  jar to get food inside

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Movement

• Move in many different speeds
• Snails secrete mucus and move slowly over the
  surface using a rippling motion of the foot
• Octopus uses a form of jet propulsion, drawing
  water into its mantle and forcing it out the
  siphon

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Reproduction

• Reproduce in many different ways
• Snails and two-shelled mollusk reproduce sexually
  by external fertilization
• Some mollusk are hermaphrodites

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Groups of Mollusks

• Gastropods
• Shell-less or single-shelled mollusks that move
  by using a muscular foot located on the ventral
  side
• Bivalves
• Have two shells that are held together by one or
  two powerful muscles
• Cephalopods
• Soft-bodied mollusks in which the head is
  attached to a single foot the foot is divided
  into tentacles or arms

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Ecology of Mollusks

• Feed on plants, prey on animals, and clean up
  their environment by filtering algae out of the
  water or by eating detritus
• Filter-feeding bivalves can be used to monitor
  water quality
• Serve as subjects of biological research