acoelomate bilateral animals

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acoelomate bilateral animals

  1. 1. Flatworms, Mesozoans, andRibbon Worms Chapter 14
  2. 2. Bilateria  Most animals have bilateral symmetry.  The vast majority of animal species belong to the clade Bilateria, which consists of animals with bilateral symmetry and triploblastic development.
  3. 3. Bilateral Symmetry  Radiallysymmetrical animals have the world coming at them from all directions.  They can catch prey coming from any direction.  Animals that begin to move about actively seeking food need a different body organization.  Distinct head end with sensory structures.  Cephalization
  4. 4. Bilateral Symmetry  Animals with bilateral symmetry have a distinct head end and can be divided into right and left halves.
  5. 5. Acoelomate Bilateral Animals  Animals that have no space between their gut and body wall are said to be acoelomate.  These animals are also triploblastic – they have three embryonic germ layers.  Organ-system level of organization – more division of labor among their organs.
  6. 6. Acoelomates  Although flatworms undergo triploblastic development, they are acoelomates.
  7. 7. Acoelomates  Theseacoelomate phyla are protostomes and have spiral cleavage.  Most have determinate cleavage.  These are the simplest animals with an excretory system.  Acoelomate phyla belong to the superphylum Lophotrochozoa
  8. 8. Phylum Acoelomorpha  Group contains ~350 species.  Members were formerly in Class Turbellaria within phylum Platyhelminthes Small flat worms less than 5 mm in length.  Typically live in marine sediments; few are pelagic.  Some species live in brackish water.  Most symbiotic but some parasitic.  Have a cellular ciliated epidermis.  Parenchyma layer contains small amount of ECM and circular, longitudinal, and diagonal muscles.
  9. 9. Phylum Acoelomorpha - Digestionand Nutrition  Incomplete digestive system - no anus.  In many acoels, the gut and pharynx are absent.  Phagocytotic cells digest food intracellularly when food is passed into temporary spaces.
  10. 10. Phylum Acoelomorpha -Reproduction  Monoecious  Female produces yolk-filled, endolecithal eggs.  Following fertilization some or all cleavage events produce a duet-spiral pattern of new cells.  May be a defining character for acoelomorphs.
  11. 11. Phylum Platyhelminthes  Members of phylum Platyhelminthes live in marine, freshwater, and damp terrestrial habitats.
  12. 12. Phylum Platyhelminthes  Flatworms are flattened dorsoventrally and have a gastrovascular cavity.  Extracellular digestion.  Undigested food is egested through the pharynx.
  13. 13. Phylum Platyhelminthes  The osmoregulatory system consists of protonephridia (excretory or osmoregulatory organs closed at the inner end) with flame cells.  Most metabolic wastes removed by diffusion across the body wall.
  14. 14. Phylum Platyhelminthes  The nervous system consists of a ladder-like network of nerves and a bilobed brain.  Many have large ocelli – light sensing organs.
  15. 15. Phylum Platyhelminthes  Manycan reproduce asexually as well as sexually.  Asexual reproduction via fission.  Sometimes the new individuals remain attached – chains of zooids.  Monoecious
  16. 16. Taxonomy  Flatworms (phylum Platyhelminthes) are divided into four classes:  Class Turbellaria – ex. Planaria  Not monophyletic  Class Trematoda – parasitic flukes  Class Monogenea – parasitic monogenetic flukes  Class Cestoda - tapeworms
  17. 17. Phylum Platyhelminthes
  18. 18. Class Turbellaria  Turbellarians are nearly all free-living and mostly marine.
  19. 19. Class Turbellaria  The best-known turbellarians, commonly called planarians, have light-sensitive eyespots and centralized nerve nets.
  20. 20. Class Trematoda  Trematodes live as parasites in or on other animals.  They parasitize a wide range of hosts.
  21. 21. Class Trematoda  Subclass Digenea, digenetic flukes, have a complex life cycle with a mollusc (snail) as the first host and a vertebrate as the final, or definitive, host.
  22. 22. Class Monogenea  Allmonogeneans are parasites.  Often found in the gills or external surfaces of fishes.
  23. 23. Class Cestoda  Tapeworms (Class Cestoda) are also parasitic and lack a digestive system.  The scolex is equipped with suckers and hooks for attachment to the host.  Each proglottid contains a set of reproductive organs.
  24. 24. Class Cestoda  Cestodes usually require at least two hosts.  Adult cestodes are parasites in the digestive tracts of vertebrates.
  25. 25. Phylum Mesozoa  Phylum Mesozoa is considered a “missing link” between protozoa and metazoa.  Have a simple level of organization.  Minute, ciliated, and wormlike animals.  All live as parasites in marine invertebrates.  Most composed of only 20 to 30 cells arranged in two layers.  Layers are not homologous to germ layers of other metazoans.  Two classes, Rhombozoa and Orthonectida, are so different that some authorities place them in separate phyla.
  26. 26. Phylum Mesozoa Rhombozoans live in kidneys of benthic cephalopods. Adults called vermiforms and are long and slender. Inner, reproductive cells give rise to vermiform larvae. When overpopulated, reproductive cells develop into gonad-like structures producing male and female gametes. Larvae are shed with host urine into the seawater.
  27. 27. Phylum Mesozoa  Orthonectids parasitize a variety of invertebrates.  Reproduce sexually and asexually.  Asexual reproduction consists of a multinucleated mass called a plasmodium.
  28. 28. Phylogeny of Mesozoans  Some consider these organisms primitive flatworms and place them in phylum Platyhelminthes.  Molecular evidence groups them with flatworms in superphylum Lophotrochozoa.  However, molecular phylogeny that included an orthonectid and two species from a rhombozoan subgroup, the dicyemids, did not show members of the two classes to be sister taxa.  The phylum may not be monophyletic.
  29. 29. Phylum Nemertea  Ribbonworms, phylum Nemertea, use a proboscis to capture prey.  Almost completely marine.  Active predators.  General body plan similar to turbellarians.
  30. 30. Phylum Nemertea  An anus is present providing these worms with a complete digestive system.  Nermeteans are the simplest animals to have a closed loop blood-vascular system.
  31. 31. Phylogeny A planuloid ancestor (like the planula larva of cnidarians?) may have given rise to a branch of descendents that were sessile or free floating and radial Cnidaria.  Another branch acquired a creeping habit and bilateral symmetry Bilateria.

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