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Acoelomate Bilateral Animals Chapter 14 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. Bilateral Symmetry

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Acoelomate Bilateral Animals

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  • 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.
bilateral symmetry
Bilateral Symmetry
  • Radially symmetrical 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
bilateral symmetry4
Bilateral Symmetry
  • Animals with bilateral symmetry have a distinct head end and can be divided into right and left halves.
acoelomate bilateral animals5
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.
  • Although flatworms undergo triploblastic development, they are acoelomates.
  • These acoelomate 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
phylum acoelomorpha
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.
phylum acoelomorpha digestion and nutrition
Phylum Acoelomorpha - Digestion and 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.
phylum acoelomorpha reproduction
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.
phylum platyhelminthes
Phylum Platyhelminthes
  • Members of phylum Platyhelminthes live in marine, freshwater, and damp terrestrial habitats.
phylum platyhelminthes12
Phylum Platyhelminthes
  • Flatworms are flattened dorsoventrally and have a gastrovascular cavity.
    • Extracellular digestion.
    • Undigested food is egested through the pharynx.
phylum platyhelminthes13
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.
phylum platyhelminthes14
Phylum Platyhelminthes
  • The nervous system consists of a ladder-like network of nerves and a bilobed brain.
  • Many have large ocelli – light sensing organs.
phylum platyhelminthes15
Phylum Platyhelminthes
  • Many can reproduce asexually as well as sexually.
    • Asexual reproduction via fission.
    • Sometimes the new individuals remain attached – chains of zooids.
  • Monoecious
  • 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
class turbellaria
Class Turbellaria
  • Turbellarians are nearly all free-living and mostly marine.
class turbellaria19
Class Turbellaria
  • The best-known turbellarians, commonly called planarians, have light-sensitive eyespots and centralized nerve nets.
class trematoda
Class Trematoda
  • Trematodes live as parasites in or on other animals.
  • They parasitize a wide range of hosts.
class trematoda21
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.
class monogenea
Class Monogenea
  • All monogeneans are parasites.
    • Often found in the gills or external surfaces of fishes.
class cestoda
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.
class cestoda24
Class Cestoda
  • Cestodes usually require at least two hosts.
    • Adult cestodes are parasites in the digestive tracts of vertebrates.
phylum mesozoa
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.
phylum mesozoa26
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.
phylum mesozoa27
Phylum Mesozoa
  • Orthonectids parasitize a variety of invertebrates.
  • Reproduce sexually and asexually.
    • Asexual reproduction consists of a multinucleated mass called a plasmodium.
phylogeny of mesozoans
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.
phylum nemertea
Phylum Nemertea
  • Ribbon worms, phylum Nemertea, use a proboscis to capture prey.
    • Almost completely marine.
    • Active predators.
    • General body plan similar to turbellarians.
phylum nemertea30
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.
  • 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