Reminder ...

• Lecture exam in ~ two weeks (March 9)
• Format:
  • 5 multiple choice (may be 0-5 correct answers) = 25 points
  • 2 short essay @ 15 points = 30 points
  • 10 fill in blanks (one or more words, short phrases) = 20 points
  • 5 short answer @ 5 points = 25 points
• I'll post a few sample questions
• I'll be in the Union from 1:15-3:00 on March 7
• Errors in readings
• Text Figure 2-9: Cnidarian should not have mesoderm
• Lab manual preface #2. Should start "By 540 mya "

Origin and Diversification of Bilateria

Early Animal Evolution

• Prior to the "Cambrian Explosion" ~ 540 mya
• Poriferans
• Cnidarians
• Ediacaran fauna
• Trace fossils

The Cambrian Explosion

• Most modern phyla suddenly burst upon the scene within about 30 million years! But probably were present long before.
• Trace fossils attributed to bilateria
• Molecular evidence suggests protostome/deuterostome split may be 700 million to over a billion years old

Implications of the fossil record

• Bilateria very old (0.7 ~ > 1 billion years)
• Small, inconspicuous for a very long time
• Relative "stasis" since early Cambrian
• A common pattern - e.g., mammals

   

Why the apparent sudden diversification?
Probably some combination of ...

• Intrinsic factors
  • Prior diversification (was not sudden)
  • Increase in size, mobility
  • Increasing importance of predators (shells)
  • Increased complexity
• Extrinsic factors
  • Increased O2 levels
  • Ediacaran extinction
  • Increased carbonate (shells)

Origins of Bilateria

• Possibly arose via "Paedomorphosis"
  • The retention of ancestrally juvenile characteristics in a sexually mature descendant
• E.g., from planula-likeancestor

The Bilateria are Triploblastic

• Bilateral symmetry (top, bottom, left, right)
• Cephalization
  (concentration of sensory organs and food handling in anterior)
• 3 tissue layers
  (mesoderm)

Advances of triploblastic animals

• Bilateral symmetry
• Cephalization
• 3 tissue layers (mesoderm)
• Organs
  • Gas exchange*
  • Nutrient transport*
  • Waste removal - protonephridia (flame cells)*
  • More efficient digestion
  • More elaborte reproduction
  • More sensitive sensory reception
• Organs usually are developed into
  systems (nervous, reproductive, digestive etc.)
  • *may be very simple

Advances of triploblastic animals

• Bilateral symmetry
• Cephalization
• 3 tissue layers (mesoderm)
• Organs
• Systems
• Greater size and higher metabolic rate!

The general formation of Mesoderm

• "The most important event in life is not birth or marriage, but rather gastrulation"

The role of Mesoderm

• Allows complex development
• Interacts with other tissues
• Formation of complex organs
• Increased muscularity (muscle is derived from mesoderm)
• Improves locomotion
• Supports organs

Two Great Bilaterian Clades

Protostome characteristics

• Spiral cleavage
• Determinate cleavage
• Blastopore forms mouth
• Mesoderm arises via schizocoely

Deuterostome characteristics

• Radial Cleavage
• Indeterminate cleavage
• Blastopore forms anus
• Mouth develops secondarily
• Mesoderm arises via enterocoely

"Protostomes" vs. "Deuterostomes"

• Protostomes
• Spiral cleavage

• Deuterostomes
• Radial cleavage

Protostomes vs. Deuterostomes

• Protostomes
• Determinate development
  (Mosaic)

• Deuterostomes
• Regulative (Indeterminate) development

Protostomes vs. Deuterostomes

• Protostomes
• Mesoderm from mesemchyme
• Blastopore forms mouth

• Deuterostomes
• Mesoderm from epithelium
• Blastopore forms anus

   

Coelom via Enterocoely

Grades and Clades
Triploblastic body plans

• Evolution of body plans very complex
• Three major grades of organization
• Acoelomates - no cavity present
• Pseudocoelomates - Cavity present between endoderm and mesoderm; from blastocoel
• Eucoelomates - Cavity present within mesoderm
• All three plans found in Protostomes
• All Deuterostomes are Eucoelomates

Acoelomate body plan

• Simplest arrangement
• Sometimes considered primitive for Bilateria
• Disadvantages
• Slow rate of diffusion
• Restricts internal organs

"Acoelomates"

• May not be monophyletic (may be independently derived)
• Phylum Platyhelminthes
• Flatworms
• Phylum Nemertea
• Ribbon worms
• Phylum Gnathostomulida
• Jaw worms

Phylum Platyhelminthes
-- the flatworms

• Successful free living and parasitic species
• Flattened body
• Incomplete gut - no anus
• Protonephridia

Phylum Platyhelminthes - Anatomy

Phylum Platyhelminthes

• Four Classes

Class "Turbellaria" - Free living flatworms or turbellarians

• Paraphyletic
• Includes Dugesia (common planarian) and several other orders
• Usually free-living (fresh water, marine, interstitial)
• Covered with ciliated epidermis
• Mouth ventral
• Gut may be absent in some interstitial forms --much like a planula

Class Trematoda - Digenetic Flukes

• All endoparasitic
• Two or more species of hosts - medically important
• Oral and ventral suckers
• No hooks
• Anterior Mouth
• Bilobed gut
• Syncytial tegument
• Egg machines

Class Monogenea - Monogenetic Flukes

• Typically ectoparasitic
• One species of host
• Syncytial tegument
• Posterior attachment organ
• Hooks
• Anterior Mouth
• Bilobed gut

Class Cestoda - Tapeworms

• All Endoparasitic - usually intestine; two or more hosts -- medically important
• Syncytial tegument
• Anterior attachment
• Hooks and suckers
• No digestive tract
• Absorption across body surface
• Asexually produced proglottids -- egg machines

Class Cestoda - Tapeworms

• All Endoparasitic -- usually intestine
• Two or more hosts -- medically important
• Syncytial tegument
• Anterior attachment
• Hooks
• No digestive tract
• Absorption across body surface
• Asexually produced proglottids -- egg machines
• Juvenile forms form a cyst or "bladder", some migrate out of intestine into other tissues

   

Pseudocoelomates = "Aschelminthes"

• Possibly Polyphyletic
• Fluid filled cavity not surrounded by mesoderm (endoderm one side, mesoderm other side)
• Numerous phyla, including:
• Phylum Rotifera
• Phylum Nematoda

"Pseudocoelomate" body plan

• Pseudocoelom
• Remnant of Blastocoel

"Pseudocoelomate" body plan

• Fluid filled body cavity
• Advantages
• Hydrostatic skeleton
• Easier internal transport
• Storage of materials
• Gamete maturation site
• Enlargement of organs
• Disadvantages
• Internal organs not well supported
• No muscles on digestive tract
• Little interaction between mesoderm and endoderm

Phylum Rotifera

• Abundant in fresh water
• Characteristics
• Corona
• Muscular Mastax
• Usually parthenogenetic
• Sexual reproduction rare

Phylum Nematoda

• Extremely successful
• Can be free-living or parasitic
• Includes C. elegans

Nematode characteristics

• Cylindrical, tapered
• Flexible cuticle
• Relative lack of cilia, flagella
• No circular muscles in body wall, only longitudinal
• Dioecious

"Eucoelomates"

• Polyphyletic
• Two Major Groups
• Protostomes
• Deuterostomes
• Both very successful

"Eucoelomate" body plan

• True coelom
• Surrounded by mesoderm
• Advantages. Same as pseudocoelom, plus:
• Better interaction of mesoderm, endoderm
• Better support of organs
• Better digestion

Eucoelomate Protostomes

• Monophyletic
• Three main phyla
• Mollusks
• Annelids
• Arthropods
• Numerous smaller phyla

Eucoelomate Protostomes

• Monophyletic
• Three main phyla
• Mollusks
• Annelids
• Arthropods
• Numerous smaller phyla

Eucoelomate Deuterostomes

• Monophyletic
• Three main phyla
• Echinodermata
• Hemichordata
• Chordata

Relationships of the Bilateria - review

Relationships of Major Animal Groups

Bilateria

• Bilateral Symmetry
• Cephalization
• Organ level of organization
• Three embryonic layers
• Ectoderm
• Endoderm
• Mesoderm

Acoelomates

• Solid mass of mesodermal tissue
• Movements tend to be slow
• Flatworms and ribbon worms

Pseudocoelomates

• Fluid filled coelom between mesoderm and endoderm
• Improved locomotion
• Space for organs
• Circulation of materials
• Found in roundworms and rotifers

Ecoelomates

• Fluid filled cavity within mesoderm
• Several advantages
• Includes most animals
• Independent origin in
• Protostomes
• Deuterostomes