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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



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


  • 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


  • 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


  • 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


Gnathiferans and Smaller Lophotrochozoans



  • 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


  • 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