Acoelomate bilateral animals
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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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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.


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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


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Bilateral Symmetry

  • Animals with bilateral symmetry have a distinct head end and can be divided into right and left halves.


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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.


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Acoelomates

  • Although flatworms undergo triploblastic development, they are acoelomates.


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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


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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.


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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.


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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.


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Phylum Platyhelminthes

  • Members of phylum Platyhelminthes live in marine, freshwater, and damp terrestrial habitats.


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Phylum Platyhelminthes

  • Flatworms are flattened dorsoventrally and have a gastrovascular cavity.

    • Extracellular digestion.

    • Undigested food is egested through the pharynx.


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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.


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Phylum Platyhelminthes

  • The nervous system consists of a ladder-like network of nerves and a bilobed brain.

  • Many have large ocelli – light sensing organs.


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Phylum Platyhelminthes

  • Many can reproduce asexually as well as sexually.

    • Asexual reproduction via fission.

    • Sometimes the new individuals remain attached – chains of zooids.

  • Monoecious


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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


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Class Turbellaria

  • Turbellarians are nearly all free-living and mostly marine.


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Class Turbellaria

  • The best-known turbellarians, commonly called planarians, have light-sensitive eyespots and centralized nerve nets.


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Class Trematoda

  • Trematodes live as parasites in or on other animals.

  • They parasitize a wide range of hosts.


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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.


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Class Monogenea

  • All monogeneans are parasites.

    • Often found in the gills or external surfaces of fishes.


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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.


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Class Cestoda

  • Cestodes usually require at least two hosts.

    • Adult cestodes are parasites in the digestive tracts of vertebrates.


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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.


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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.


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Phylum Mesozoa

  • Orthonectids parasitize a variety of invertebrates.

  • Reproduce sexually and asexually.

    • Asexual reproduction consists of a multinucleated mass called a plasmodium.


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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.


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Phylum Nemertea

  • Ribbon worms, phylum Nemertea, use a proboscis to capture prey.

    • Almost completely marine.

    • Active predators.

    • General body plan similar to turbellarians.


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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.


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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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