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CHAPTER 8. Acoelomates. General Features. Animals that actively seek food, shelter, home sites, and mates require a different set of strategies and body organization than radially symmetrical sessile organisms Two major evolutionary advances Cephalization

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chapter 8

CHAPTER 8

Acoelomates

general features
General Features
  • Animals that actively seek food, shelter, home sites, and mates require a different set of strategies and body organization than radially symmetrical sessile organisms
  • Two major evolutionary advances
    • Cephalization
      • Concentrating sense organs in the head region
    • Primary bilateral symmetry
      • Body can be divided along only 1 plane of symmetry to yield 2 mirror images of each other
general features1
General Features
  • Triploblastic
  • Acoelomates
    • Typical acoelomates have only one internal space, the digestive cavity
      • Region between the epidermis and digestive cavity is filled with parenchyma
phylum acoelomorpha
Phylum Acoelomorpha

Characteristics

  • Small flat worms less than 5 mm in length
  • Typically live in marine sediments; few are pelagic
  • Have a cellular ciliated epidermis
  • Parenchyma layer contains small amount of ECM and circular, longitudinal, and diagonal muscles
phylum platyhelminthes
Phylum Platyhelminthes

Characteristics

  • Commonly called flatworms
  • Vary from a millimeter to many meters in length
  • Some free-living; others parasitic
  • The parasitic clades
    • Share an external body covering called a syncytial tegument or neodermis
phylum platyhelminthes1
Phylum Platyhelminthes
  • Platyhelminthes is divided into four classes:Turbellaria, Trematoda, Monogenea, and Cestoda
    • Class Turbellaria
      • Mostly free-living forms
      • Most are bottom dwellers in marine or freshwater
      • Freshwater planarians
        • Found in streams, pools, and hot springs
      • Terrestrial flatworms limited to moist places
    • All members of Monogenea and Trematoda (flukes)and Cestoda (tapeworms) are parasitic
phylum platyhelminthes2
Phylum Platyhelminthes

Form and Function

  • Epidermis and Muscles
    • Most have cellular, ciliated epidermis on a basement membrane
    • Rod-shaped rhabdites
      • Swell and form a protective mucous sheath
    • Most turbellarians have dual-gland adhesive organs
      • Viscid gland cells fasten microvilli of anchor cells to substrate
      • Secretions of releasing gland cells provide a quick chemical detachment
phylum platyhelminthes3
Phylum Platyhelminthes

Nutrition and Digestion

  • Cestodes have no digestive system
  • Others have a mouth, pharynx, and intestine

Excretion and Osmoregulation

  • Flatworms have protonephridia
    • Used for osmoregulation
    • Wall of the duct beyond the flame cell bears folds or microvilli to resorb ions and molecules
    • Majority of metabolic wastes
      • Removed by diffusion across the cell wall
    • Collecting ducts join and empty at nephridiopores
  • Marine turbellarians
    • Lack these units
    • No need to expel excess water
phylum platyhelminthes4
Phylum Platyhelminthes

Sense Organs

  • Active locomotion favored cephalization and evolution of sense organs
  • Ocelli (light-sensitive eyespots)
    • Present in turbellarians, monogeneans, and larval trematodes
  • Tactile and chemoreceptive cells
    • Abundant, especially in the ear-shaped auricles
  • Statocysts (equilibrium) and rheoreceptors (sense direction of water currents) in some
  • Sensory nerve endings found in
    • Oral suckers and genital pores of parasitic groups
phylum platyhelminthes5
Phylum Platyhelminthes

Reproduction and Regeneration

  • Fission
    • Many turbellarians constrict behind the pharynx and separate into two animals
    • Each half regenerates the missing parts
      • Provides for rapid population growth
    • Some do not separate immediately, creating chains of zooids
  • Regeneration
    • If the head and tail are cut off
      • Each end grows the missing part; it retains polarity
    • Extract of heads added to a culture of headless worms prevents regeneration
phylum platyhelminthes6
Phylum Platyhelminthes
  • Some asexual reproduction occurs in intermediate hosts
  • Nearly all are monoecious but cross-fertilize
  • Endolecithal eggs
  • Some turbellarians and all other groups have female gametes with little yolk
    • Yolk is contributed by separate organs, vitellaria
    • Vitelline ducts bring yolk cells to the zygote (ectolecithal development)
    • Zygote and yolk cells surrounded by eggshell move into the uterus
phylum platyhelminthes7
Phylum Platyhelminthes
  • Larval trematodes emerge as ciliated larvae
    • Penetrate a snail or eaten by a host
  • Cestodes hatch only after being consumed by a host
    • Many different animals can serve as intermediate hosts
phylum platyhelminthes8
Phylum Platyhelminthes

Classification of Phylum Platyhelminthes

  • Class Turbellaria
  • Class Trematoda
  • Class Monogenea
  • Class Cestoda
phylum platyhelminthes9
Phylum Platyhelminthes

Class Turbellaria

  • Mostly free-living
  • Range from 5 mm to 50 cm long
  • Others move by cilia
    • Glide over a slime track secreted by adhesive glands
    • Rhythmical muscular waves pass backward from the head
phylum platyhelminthes10
Phylum Platyhelminthes

Class Trematoda

  • All trematodes are parasitic flukes
  • Most adults are endoparasites of vertebrates
  • They resemble ectolecithal turbellaria but the tegument lacks cilia in adults
  • Adaptations for parasitism include:
    • Penetration glands
    • Glands to produce cyst material
    • Hooks and suckers for adhesion
    • Increased reproductive capacity
phylum platyhelminthes11
Phylum Platyhelminthes

Sheep Liver Fluke

  • Fasciola hepatica
    • First digenean whose life cycle was described
  • Adult fluke lives in bile passageways in the liver of sheep and other ruminants
  • Eggs are pass out in feces
  • Miracidia hatch and penetrate snails to become sporocysts
  • After two generations of rediae
    • Cercaria encyst on vegetation and await being eaten by sheep
  • When eaten, metacercariae develop into young flukes
phylum platyhelminthes12
Phylum Platyhelminthes

Clonorchis sinensis Life Cycle

  • Adults live in bile passageways of humans and other fish-eating mammals
  • Eggs containing a complete miracidium are shed into water with feces
  • The eggs hatch only when ingested by snails of specific genera
  • Miracidium enters snail tissue and transforms into a sporocyst
  • Sporocyst produces one generation of rediae, which begin differentiation
phylum platyhelminthes13
Phylum Platyhelminthes
  • Rediae pass into the snail liver
    • Continue embryonation into tadpole-like cercariae
  • Cercariae escape into water
    • Make contact a fish in the family Cyprinidae
    • Bore into fish muscles or under scales
    • Shed tail and encyst as metacercariae
  • A mammal eats raw fish
    • Cyst dissolves and flukes migrate up bile duct
  • Heavy infection can destroy the liver and result in death
  • Control of parasites
    • Destroy snails and thoroughly cook fish
phylum platyhelminthes14
Phylum Platyhelminthes

Schistosoma Life Cycle

  • Eggs discharged in human feces or urine
  • In water, eggs hatch as ciliated miracidia
  • Must contact a particular species of snail to survive
  • In the snail, they transform to sporocysts
  • Sporocysts produce cercaria directly
  • Cercariae escape the snail and swim until they contact bare human skin
  • Cercariae pierce the skin and shed their tails
phylum platyhelminthes15
Phylum Platyhelminthes
  • Enter blood vessels and migrate to the hepatic portal blood vessels
  • Develop in the liver and they migrate target sites
  • Eggs released by females are extruded through gut or bladder lining and exit with feces or urine
  • Eggs that remain behind become centers of inflammation
phylum platyhelminthes16
Phylum Platyhelminthes

Class Monogenea

  • External parasites of fish, especially gills, but a few are found in bladders of frogs and turtles
  • Have a single host
  • Posterior hooks may become the posterior attachment organ of the adult, the opisthaptor
  • Opisthaptors vary widely (hooks, suckers, clamps)
    • Withstand the force of water flow
  • Some serious economic problems in fish farming
phylum platyhelminthes17
Phylum Platyhelminthes

Class Cestoda

  • Tapeworms have long flat bodies with scolex
    • Holdfast structure with suckers and hooks
  • Scolex is followed by a linear series of reproductive units or proglottids
  • Lack a digestive system
  • Tegument is syncytial and has no cilia
  • Entire surface of cestodes is covered with projections (microtriches) similar to microvilli seen in the vertebrate small intestine
    • Microtriches increase the surface area for food absorption
phylum platyhelminthes18
Phylum Platyhelminthes

Taenia saginata: Beef Tapeworm

  • Lives as an adult in the digestive system of humans
  • Juvenile form found in intermuscular tissue of cattle
  • Mature adults can reach over 10 meters in length with over 2000 proglottids
  • Gravid proglottids (with shelled, infective larvae) pass in feces
phylum platyhelminthes19
Phylum Platyhelminthes

Life Cycle

  • Cattle swallow shelled larvae that hatch as oncospheres
  • Oncospheres use hooks to burrow through the intestinal wall into blood or lymph vessels
  • When they reach voluntary muscle, they encyst to become bladder worms (cysticerci)
  • When the infected meat is eaten, the cyst wall dissolves and the scolex attaches to intestinal mucosa
phylum mesozoa
Phylum Mesozoa
  • 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
phylum nemertea
Phylum Nemertea

Characteristics

  • Often called ribbon worms
  • Have a long muscular tube, the proboscis
  • Most are less than 20 cm long
  • Proboscis is an eversible organ
    • Protruded from a rhynchocoel for defense and catching prey
  • Proboscis is everted by fluid pressure and retracted by muscles
  • Has a sharp-pointed stylet at the tip
phylum nemertea1
Phylum Nemertea

Feeding and Digestion

  • Carnivorous: Feed on dead or living prey
  • Slime-covered proboscis wraps around prey
  • Stylet pierces and holds prey until it is thrust into mouth
  • Pours a neurotoxin, tetrodotoxin (the toxin in puffer fishes) on its prey
phylum nemertea2
Phylum Nemertea

Excretion and Respiration

  • Near the edge of body is a lateral tube with branches and flame cells
  • Wastes picked up from parenchymal spaces by flame cells are carried out excretory ducts
  • Protonephridia are so closely associated with circulatory system that they are truly excretory rather than simply osmoregulatory in function as in flatworms
  • Respiration occurs through the body surface
chapter 9

CHAPTER 9

Gnathiferans and Smaller Lophotrochozoans

protostomia
Protostomia

Phylogeny

  • Large clade sometimes called a superphylum
  • Protostome phyla are divided between two large clades
    • Lophotrochozoa and Ecdysozoa
  • Lophotrochozoa (10 Phyla)
    • Ancestors possessed complex cuticular jaws
      • Gnathostomulida
      • Micrognathozoa
      • Rotifera
      • Acanthoephala
protostomia1
Protostomia
  • 6 other lophotrochozoan phyla
    • Gastrotricha
      • Tiny aquatic animals that may be closely related to gnathiferans
    • Molecular characteristics place the following with Lophotrochozoa
      • Cycliophora
      • Entoprocta
      • Ectoprocta
      • Brachiopoda
      • Phoronida
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