Acoelomate Bilateral Animals

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Acoelomate Bilateral Animals
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Acoelomate Bilateral Animals

• Consist of phyla
• Phylum Platyhelminthes
• Phylum Nemertea
• And others

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Acoelomate Bilateral Animals

1. Simplest organisms to have bilateral symmetry
2. Triploblastic
3. Lack a coelom
4. Organ-system level of organization
5. Cephalization
6. 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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Review

• 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
• Cephalization
• specialization of sense organs in head end of
  animals

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• Acoelomates lack a true body cavity
• Solid body
• no cavity b/w the digestive tract and outer body
  wall

Acoelomate Phylum Platyhelminths Or not shown
here Nemerterean
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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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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Polyclad

• From Red Sea

http//www.rzuser.uni-heidelberg.de/bu6/
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Phylum Platyhelminthes
Free living

• Flatworms

Parasitic
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• From Atlantic ocean

http//www.rzuser.uni-heidelberg.de/bu6/
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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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Phylum Platyhelminthes

• Organ systems absent
• Circulatory
• Respiratory

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Phylum Platyhelminthes (contd)

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

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Class Turbellaria (contd)

• Move by muscles, ciliated epidermis

/gastrovascular cavity
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Class Turbellaria (contd)

• 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 (contd)

• Flame cells branch from tubules
• Ciliary projections drive fluid down tubule
• Tubules open to outside nephridiopore

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Class Turbellaria (contd)

• nervous system with nerve ganglion
• ganglion- aggregation of nervous tissue
• Cephalization- cerebral ganglion primitive brain

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Class Turbellaria (contd)

• Ocelli light-sensitive eyespots

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Turbellarian Reproduction

• Asexual (fission)
• transverse
• Sexual
• Monoecious (mostly)
• Cross-fertilization

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• Other 3 classes
• Class Trematoda
• Class Monogenea
• Class Cestoda

• All parasitic
• lack cilia
• Have unusual body covering tegument

• 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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• 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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Class Cestoda

• Tapeworms
• Endoparasites
• Vertebrate host
• Live in digestive tract
• 1 mm- 25m long
• (EWWWW!!)

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Tapeworm

• Tapeworms
• Are also parasitic and lack a digestive system

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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

scolex
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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 lt20cm
• Marine mud, sand
• Elongate, flattened worms

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Phylum Nemertea (contd)

• Unlike the platyhelminthes, Complete digestive
  tract, with anus
• One-way
• More efficient allows larger growth

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Nemertea
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Phylum Nemertea (contd)

• 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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Sea serpents?