Bilateral Symmetry

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Bilateral Symmetry

• The remaining phyla are all bilaterally
  symmetrical or at least have primary bilateral
  symmetry
• Also called Bilateria
• the development of bilateral symmetry is one of
  the most important traits in higher animals
• it implies that a single line or plane will
  divide the body into equal halves or mirror
  images

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Bilateral Symmetry

• bilateral symmetry has allowed several other
  important changes in body structure
• Cephalization- concentration of the nerve tissue
  into a head which allow for a single organ to
  direct the functions of the animal
• allow for greater organ development
• allows for greater differentiation of structure
  such as appendages

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Bilateral Symmetry

• Advantages of bilateral symmetry
• better coordinated movement
• much quicker and more precise response to
  stimulation since it is directed by a central
  nerve center.

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Acoelomates

• Includes phyla Platyhelminthes and Nemertea
• those organisms that do not have a true coelom or
  body cavity
• are entirely solid except for the gastric cavity
  or coelenteron

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PHYLUM PLATYHELMINTHES

• THE FLATWORMS

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CHARACTERISTICS OF PHYLUM

• bilaterally symmetrical
• triploblastic 3 distinct tissue or germ layers
• Ectoderm
• Mesoderm
• Endoderm
• Dorsoventrally flattened
• lack an anus incomplete digestive tract

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CHARACTERISTICS OF PHYLUMcont.

• coelom- solid mesenchyme (mesoderm) first phylum
  to show a definite cellular mesoderm
• have no true respiratory system or circulatory
  system
• Have cutaneous respiration
• first phylum to show distinct excretory system
  get rid of nitrogenous waste
• are usually hermaphroditic

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PLATYHELMINTHES

• They consist of 4 classes of flatworms 3 of
  which are entirely parasitic, the other
  free-living
• Turbellaria- free-living
• Monogenea- parasitic, flukes
• Trematoda- parasitic, flukes
• Cestoda- parasitic, tape worms

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CLASS TURBELLARIA
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CLASS TURBELLARIA

• consists of all of the free-living flatworms, but
  some are found on aquatic hosts as ectoparasites
  or commensals
• few live in freshwater (ie. Planaria), marine as
  well as moist terrestrial habitats

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Morphology

• the outer surface of turbellarians consists of
  ciliated epidermis
• usually best developed on the ventral surface
  and function in locomotion
• epidermis also has a large number of mucous
  glands that secrete mucous that is used by the
  cilia in locomotion - Rhabdites

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Morphology

• Important Structures
• Eyespots- for light reception phototaxis
• Mouth with pharynx
• Incomplete digestive tract
• No anus

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Locomotion/Movement

• Below epidermis are a series of muscles
• Dorso-ventral or oblique muscles (parenchymal)
• circular muscles
• longitudinal muscles
• Movement is combination of these muscles
  contracting

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Feeding

• Turbellarians primarily carnivorous
• capture of prey is done by wrapping themselves
  around it and entangling it with mucous
• they ingest the whole prey or
• suck its juices through a hardened stylet
  (modified pharynx)
• in many species the pharynx is completely
  eversible and can envelope the entire prey
• there is no anus so ingestion and egestion are
  through the mouth

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Excretion/Osmoregulation

• done by specialized cell called flame cells or
  protonephridia
• first group with specialized excretory
  structures

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Nervous System and Sensory Structures

• Ladder-like arrangement
• show a beginnings of a well developed central
  nervous system- cepahlization
• a variety of sensory cells and glands most are
  chemo- or tactile receptors
• 2 eye spots or ocelli, which can discriminate
  varying light intensities

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Reproduction

• the most complex organ system
• most are hermaphroditic
• cross fertilization is the most common mode of
  reproduction
• some self fertilization can and does occur
• usually the eggs and sperm are produced at
  different times in the individual

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Reproduction cont.

• fertilization is internal
• fertilized eggs are usually deposited in clusters
  
• winter eggs have a hard outer covering that can
  survive desiccation and freezing
• in freshwater forms eggs hatch into miniature
  adult forms direct development
• in marine forms (not all) a larva is produced
  called a Müller's larva which is free swimming

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Asexual Reproduction

• fragmentation and regeneration
• many studies have been done on regeneration in
  Planaria

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Life as a Parasite

• Loss of sensory structures
• Eye spots tactile sensors
• Loss of some organ systems
• Digestive system
• Increased reproductive potential
• Insures survival of species

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Parasites and Hosts

• Host- organism parasite lives on or in
• Definitive or Primary host- where parasite has
  sexual reproduction
• Secondary or Intermediate hosts- parasite has
  asexual reproduction
• Endoparasite/Ectoparasite- in or on host
• Facultative/Obligatory parasites
• Facultative- on host only part time e.g., leech
• Obligatory- on or in host entire life e.g., fluke

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Classes Trematoda and Monogenea(Flukes)

• all are parasitic (endoparasitic and
  ectoparasitic)
• most parasitic to vertebrates especially fish
• most have intermediate hosts as invertebrates
• many species are economically and medically
  important

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Characteristics of Flukes

• Have many of the same characters as turbellarians
• usually have an anterior sucker around mouth and
  a posterior sucker- used to attach to host tissue
• suckers best developed in Monogenea where they
  are called Opisthaptors
• body does not have ciliated epidermis as
  turbellarians

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Reproduction of Flukes

• Sexual in definitive host
• is generally through copulation with cross
  fertilization sometimes self fertilization
  occurs
• Asexual in intermediate hosts

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Typical Life Cycle

• Egg
• Miracidium larva
• Sporocysts
• Redia
• Cercaria
• Metacercaria
• Adult
• Usually 2 but as
• many as 4 hosts
• Often intermediate host is a snail

Two intermediate hosts
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Another Example this one with one intermediate
host
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CLASS CESTODA(tapeworms)

• All are endoparasites
• the body is covered by a cuticle like the
  trematodes
• they differ from all of the other flatworms in
  that they do not have a digestive tract

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Characteristics of Tapeworms

• Features similar to other flatworms
• E.g. respiration, and excretion
• Morphological features include
• Scolex
• Rostellum
• Suckers and hooks
• Proglottids (repeated segments)
• Immature sexually immature
• Mature sexual structures present
• Gravid filled with fertilized eggs

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Typical Life Cycle

• Egg
• Onchoshere larva
• Cysticercus larva in cysts
• Adult
• Usually only 2 hosts
• Intermediate host usually warm blooded

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Hydatid cysts can form when larva is ingested by
wrong host example here is in liver of human
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Example here is dog tapeworm hydatid cysts in
brain of human, which are often inoperable and
fatal
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PHYLUM NEMERTEA

• THE PROBOSCIS OR RIBBON WORMS

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NEMERTEA

• Almost all are marine, one freshwater genus and
  one terrestrial genus
• most are free-living, bottom dwellers

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PHYLUM NEMERTEA

• closely related to flatworms but differ in a
  number of ways
• have a circulatory system
• have tubular complete (first phylum with
  complete gut) gut- mouth and anus
• have an eversible proboscis

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PHYLUM NEMERTEA

• the most diagnostic feature is the eversible
  proboscis
• used to capture small prey or for browsing on
  dead and decaying organisms are carnivorous
• lies in a fluid filled cavity (rhyncocoel)
• in some species the proboscis is armed with
  barbs or spines and may inject a toxin
• once food is captured the food is passed into the
  mouth and gut

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REPRODUCTION

• are dioecious and fertilization is external
• reproductive organs are simple masses of
  mesenchymal cells
• eggs and sperm are released to outside through
  them
• Produce a free living larva- Pilidium
• in some species fragmentation is common
  especially when worms are disturbed
• usually only anterior end can regenerate to
  produce a new posterior end

Pilidium larva
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OTHER FEATURES

• Excretion- Flame cells
• Respiration- osmosis
• Locomotion- gliding
• Feeding- proboscis, ingestion of prey
• Regeneration