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Acoelomate Bilateral Animals. Acoelomate Bilateral Animals. Consist of phyla: Phylum Platyhelminthes Phylum Nemertea And others. Reproductive and osmoregulatory systems. Acoelomate Bilateral Animals. Simplest organisms to have bilateral symmetry Triploblastic Lack a coelom

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Acoelomate Bilateral Animals

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Acoelomate bilateral animals l.jpg

Acoelomate Bilateral Animals

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Acoelomate Bilateral Animals

  • Consist of phyla:

    • Phylum Platyhelminthes

    • Phylum Nemertea

    • And others

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Reproductive and osmoregulatory systems

Acoelomate Bilateral Animals

  • Simplest organisms to have bilateral symmetry

  • Triploblastic

  • Lack a coelom

  • Organ-system level of organization

  • Cephalization

  • Elongated, without appendages

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Bilateral Symmetry

  • Divided along sagittal plane into two mirror images

    • sagittal= divides bilateral organisms into right and left halves

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  • Anterior= head end

  • Posterior= tail end

  • Dorsal= back side

  • Ventral= belly side

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Bilateral animals

  • Bilateral symmetry = important evolutionary advancement

    • Important for active, directed movement

      • Anterior, posterior ends

    • One side of body kept up (dorsal) vs. down (ventral)

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Directed movement evolved with anterior sense organscephalization


  • specialization of sense organs in head end of animals

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Phylum Platyhelminths

Or not shown here


  • Acoelomates lack a true body cavity

    • Solid body

    • no cavity b/w the digestive tract and outer body wall

This is a round worm

Different Phylum

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Acoelomates are triploblastic

  • Triploblastic (3 germ layers)

    • Germ layer= layers in embryo that form the various tissues and organs of an animal body

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3 germ layers

  • Ectoderm

    • Outermost germ layer

    • Gives rise to outer covering of animal ie. epidermis

  • Endoderm

    • Innermost germ layer

    • Gives rise to inner lining of gut tract

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  • Mesoderm

    • Middle germ layer

    • b/w ectoderm and endoderm

    • Gives rise to various tissues/organs (ie. muscles)

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Acoelomate animals have an organ-system level of organization

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Digestive tract and nervous system

Acoelomate animals have an organ-system level of organization

  • Organ-system

    • Different organs operate together (ie. excretory system, nervous system)

    • mesodermal tissue gives rise to parenchyma

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  • From Red Sea

Phylum platyhelminthes l.jpg

Phylum Platyhelminthes


Free living


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  • From Atlantic ocean

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Phylum Platyhelminthes

  • Flattened dorsoventrally

    • flatworms

  • 34,000 species

  • Gastrovascular cavity (if present) has only one opening (mouth = anus)

  • Mostly monoecious

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Phylum Platyhelminthes

  • First phylum that has an Organ systems present

    • derived mesodermally (parenchyma):

      • Muscular system

      • Digestive system (incomplete; gastrovascular type) (absent in some)

      • Nervous system

      • Excretory system (absent in some)

      • Reproductive system

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Rely on diffusion

Phylum Platyhelminthes

  • Organ systems absent:

    • Circulatory

    • Respiratory

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Hymenolepsis- rat tapeworm

Phylum Platyhelminthes (cont’d)

  • Divided into 4 classes:

    • Class Turbellaria (mostly free-living flatworms)

    • Class Trematoda (parasitic flukes)

    • Class Monogenea (ectoparasitic flukes)

    • Class Cestoda (tapeworms)

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Class Turbellaria

  • Mostly free-living flatworms

  • Marine (mostly) or freshwater bottom-dwellers

  • Predators and scavengers

  • First group of bilateral symmetrical animals

Planarian genus Dugesia

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Digestion is completed within

the cells lining the gastro-

vascular cavity, which has

three branches, each with

fine subbranches that pro-

vide an extensive surface area.

Pharynx. The mouth is at the

tip of a muscular pharynx that

extends from the animal’s

ventral side. Digestive juices

are spilled onto prey, and the

pharynx sucks small pieces of

food into the gastrovascular

cavity, where digestion continues.

Undigested wastes

are egested

through the mouth.




Ganglia. Located at the anterior end

of the worm, near the main sources

of sensory input, is a pair of ganglia,

dense clusters of nerve cells.

Ventral nerve cords. From

the ganglia, a pair of

ventral nerve cords runs

the length of the body.

Figure 33.10

  • The best-known turbellarians, commonly called planarians

    • Have light-sensitive eyespots and centralized nerve nets

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Class Turbellaria (cont’d)

  • Move by muscles, ciliated epidermis

/gastrovascular cavity

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Class Turbellaria (cont’d)

  • Freshwater turbellarians adapted osmoregulatory structures

    • Protonephridia

      • protos= first

      • nephros= kidney

      • network of fine tubules running down sides of organism

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Class Turbellaria (cont’d)

  • Flame cells= branch from tubules

    • Ciliary projections drive fluid down tubule

  • Tubules open to outside= nephridiopore

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Class Turbellaria (cont’d)

  • nervous system with nerve ganglion

    • ganglion- aggregation of nervous tissue

  • Cephalization- cerebral ganglion= primitive brain

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Class Turbellaria (cont’d)

  • Ocelli= light-sensitive eyespots

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Reproductive and osmoregulatory systems

Turbellarian Reproduction

  • Asexual (fission)

    • transverse

  • Sexual

    • Monoecious (mostly)

    • Cross-fertilization

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  • All parasitic

  • lack cilia

  • Have unusual body covering: tegument

Other 3 classes:

  • Class Trematoda

  • Class Monogenea

  • Class Cestoda

  • Outer zone of tegument (glycocalyx)

    • consists of proteins and carbohydrates

    • aids in transport of nutrients, waste, gases

    • Protection against host defenses

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Class Trematoda

  • Parasitic flukes

  • Endoparasites

    • Hooks, suckers, increased reproductive capacity

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  • 1mm-6cm long

  • Complex life cycle:

  • Definitive host (primary/final host)

    • where parasite matures and reproduces (sexually) (eggs released)

    • vertebrate

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  • Intermediate host

    • Mollusc (ie. snail)

    • Hosts in which larval stages develop and undergo asexual reproduction

    • Results in an increase in the number of the individuals

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Mature flukes live in the blood vessels of the human

intestine. A female fluke fits into a groove running

the length of the larger male’s body, as shown in

the light micrograph at right.




1 mm


These larvae penetrate

the skin and blood

vessels of humans

working in irrigated

fields contaminated

with infected human


Blood flukes reproduce

sexually in the human host.

The fertilized eggs exit the

host in feces.



The eggs develop in

water into ciliated

larvae. These larvae

infect snails, the

intermediate hosts.

Asexual reproduction

within a snail results in

another type of motile

larva, which escapes from

the snail host.


Snail host

Figure 33.11

  • Trematodes that parasitize humans

    • Spend part of their lives in snail hosts

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Chinese Liver Fluke

  • Infects 30 million people in eastern Asia

  • Lives in ducts of liver

    • Eats epithelial tissue, blood

  • Definitive host:

    • Humans, dogs, cats

  • 2 intermediate hosts:

    • snail

    • fish

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Class Monogenea

  • Parasitic flukes

  • Mostly ectoparasites

  • Single host, mostly fish

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Hymenolepsis- rat tapeworm

Class Cestoda

  • Tapeworms

  • Endoparasites

  • Vertebrate host

    • Live in digestive tract

  • 1 mm- 25m long


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Proglottids with

reproductive structures

200 µm




Figure 33.12


  • Tapeworms

    • Are also parasitic and lack a digestive system

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Hymenolepsis- rat tapeworm

Class Cestoda

  • Highly specialized

  • Lack mouth, digestive tract

    • Absorb nutrients across body wall

  • Hooks and suckers

    • “head”= scolex

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  • Adult tapeworms consist of long series of repeating units= proglottids

  • Chain of proglottids= strobila

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  • Tapeworms are monoecious (mostly)

    • Mostly cross-fertilization

  • No specialized sense organs


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  • Cestodes depend on host digestion

    • Small molecules in host intestine, liver

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Beef Tapeworm

  • Definitive host= human

  • Intermediate host= cattle

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Phylum Nemertea- ribbon worm

  • Triplobastic, acoelamate

  • bilateral symmetry

  • Unsegmented

  • Ciliated epidermis

  • Closed circulatory

  • usually <20cm

  • Marine mud, sand

  • Elongate, flattened worms

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Phylum Nemertea (cont’d)

  • Unlike the platyhelminthes, Complete digestive tract, with anus

    • One-way

    • More efficient; allows larger growth

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Phylum Nemertea (cont’d)

  • Cerebral ganglion, longitudinal nerve cords

  • Long proboscis used in carnivorous species

  • Two lateral blood vessels yet no heart

  • Dioecious

    • “two” “house”

    • Male and female organs in separate individuals

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