Chapter 32

							Protostome Animals
     Chapter 32
      Protostome Animals
• Protostomes are bilaterally
  symmetric, coelomate animals
• Undergo early development differently
  than deuterostomes
• Contains the most successful phylum,
  Arthropoda and some very obscure
  phyla, Echiura ( about 135 species)
• Phyla have distinct body plans,
  feeding apparatuses, and locomotions
Why Do Biologists Study
    Protostomes?
            Arthropods
• Most diverse and
  numerous phylum of
  animals
  – 925,000 known and
    identified
• Live in almost every
  know habitat
• Cause important
  diseases
 Crustaceans and Mollusks
• Crustaceans and
  mollusks are
  consumers, predators,
  and scavengers in
  many marine food
  chains
• They are among the
  most expensive and
  sought-after seafood
 Insects, Spiders, and Mites
• Insects eat about a third of the crops
  planted by farmers
• They are the most economically
  important group of protostomes
• Dominant consumers, predators, and
  scavengers in all terrestrial
  ecosystems
• Most flowering plants are pollinated by
  insects
 How Do Biologists
Study Protostomes?
  How Do Biologists Study
      Protostomes?
• Based on data
  obtained from:
  – morphology
    studies
  – the fossil record
  – molecular
    phylogenies
Protostome Early Development
•  Researchers have drawn some
   general conclusions about
   protostomes:
1. Protostomes undergo spiral
   cleavage after fertilization.
    •   During gastrulation, the initial
        invagination that forms in the embryo
        becomes the mouth.
    •   A body cavity, or coelom, may form
        within blocks of mesodermal tissue.
    Protostome Symmetry
2. Protostomes are
   bilaterally
   symmetric
  • Have a distinctive
    head and posterior
    region.
  • All protostomes
    have three
    embryonic tissues.
Protostome Segmentation
3. Some
   protostomes
   may exhibit
   segmentation
  • Their bodies
    have a series of
    compartments
    with repeated
    structures.
   Prehistoric Protostomes
4. Most
   protostome
   phyla first
   appear in the
   Burgess Shale
   faunas, dated
   at about 525–
   515 million
   years ago
 Protostomes are Monophyletic
5. Molecular
   phylogenies support
   the hypothesis that
   protostomes are a
   monophyletic group
  •   A branching event
      occurred within the
      lineage, producing
      two groups:
      •   the Lophotrochozoa
          and the Ecdysozoa.
Analyzing Morphological
         Traits
     Coelom Development
• Radical changes occurred in coelom
  formation as protostomes diversified
  – Acoelomate species arose again
  – Pseudocoelom, arose independently in
    certain protostome groups.
  – Most protostomes have bodies with a
    basic tube-within-a-tube design
  – arthropods and mollusks have
    specialized body plans where the coelom
    is drastically reduced
Tube- Within-A-Tube
• Arthropods and mulluscs have a
  hemocoel (“blood-hollow”) that
  provides space for internal organs
  and fluid circulation
    Evaluatin Protostome History
•    Two major events
     occurred in the fossil
     record of protostomes:
     1. Extinction of the
        trilobites, a marine
        arthropod, about 250
        million years ago,
     2. The appearance of
        insects about 400
        million years ago
         Evaluating Molecular
             Phylogenies
•   Molecular phylogenies support the hypothesis that
    protostomes are a monophyletic group that
    diverged into two major subgoups.
  What Themes Occur in the
Diversification of Protostomes?
    Protostome Diversification
•   Phylogenetic analyses
    suggests that repeated
    water-to-land transitions
    occurred as protostomes
    diversified
•   Two of the most important
    problems that animals had
    to solve were
    1. exchanging gases
    2. drying out
  Protostome Diversification
• Protostomes solved
  these problem in
  different ways
• The evolution of
  specialized body plans
  provided a foundation for
  diversification in the
  most species-rich
  lineages
  – The arthropods and
    mollusks
  Feeding
• A wide diversity
  of feeding
  strategies is
  reflected in the
  diversity of
  mouthparts
  found in
  protostome
  animals
               Movement
•   Variation in movement
    depends on:
    1. The presence or
       absence of limbs and
    2. the type of skeleton
       that is present
•   Several evolutionary
    innovations allowed
    protostomes to move
    in unique ways
        Jointed Limbs
1. Jointed limbs
   allow for precise
   swimming and
   running
   movements
            Wings
2. Insects were the
   first organisms
   that had wings
   and could fly
Muscular Foot
3. The muscular
   foot allowed
   mollusks to
   glide along
   surfaces
     Water Propulsion
4. The muscle-lined
   mantle allowed
   the squid to use
   jet propulsion to
   propel itself
   backward
     Reproduction and Life
           Cycles
• Can reproduce asexually via
  parthenogenesis
  – Unfertilized eggs that develop into
    offspring
• Commonly reproduce via sexual
  reproduction
• Two innovations occurred during
  protostome diversification: (1) evolution
  of metamorphosis and (2) an egg that
  would not dry out on land.
Key Lineages of Protostomes
        Lophotrochozoans
• Named for two
  distinctive
  morphological
  traits: (1) a feeding
  structure called a
  lophophore and
  (2) a type of larvae
  called a
  trochophore
   Rotifera
  (Rotifers)
• Rotifers have a
corona of cilia at
their anterior that is
used for
suspension feeding
Platyhelminthes
  (Flatworms)
• This group
  includes (1) the
  free-living
  flatworms, (2) the
  endoparasitic
  tapeworms, and (3)
  the endo- or
  ectoparasitic flukes
     Annelida (Segmented
           Worms)
• Annelids have a segmented body
  plan and a coelom that functions as a
  hydrostatic skeleton
      Mollusca (Mollusks)
• Most species-rich and
  morphologically diverse group in the
  Lophotrochozoa.
• Specialized body plan based on a
  muscular foot, a visceral mass, and a
  mantle that secretes a calcium
  carbonate shell
• Bivalves are suspension feeders
• The other three groups of mollusks
  are herbivores or predators
 Mollusca—Bivalvia (Clams,
 Mussels, Scallops, Oysters)

• The bivalves
  have two
  separate shells,
  made of calcium
  carbonate, that
  are hinged
  Mollusca—Gastropoda
(Snails, Slugs, Nudibranchs)

• Have a large
  muscular foot on
  their ventral side
  and many lack
  shells
  Mollusca—Polyplacophora
          (Chitons)

• The Greek word roots
  that inspired the name
  Polyplacophora mean
  “many-plate-bearing”
• Have eight calcium
  carbonate plates along
  their dorsal side
  Mollusca—Cephalopoda
(Squid, Nautilus, Octopuses)
• Have a well-
  developed head and
  a foot that is
  modified to form
  long, muscular
  tentacles
• They also have large
  brains and eyes with
  sophisticated lenses.
           Ecdysozoans
• All members of this
  lineage grow by
  molting—shedding
  of the exoskeleton
  or external covering
  (cuticle)
  Nematoda (Roundworms)
• Unsegmented
  worms with a
  pseudocoelom
  – Tube-within-a-tube
    body plan
• No appendages
• Common parasites
  of humans and
  plants
   Arthropoda (Arthropods)
• Most successful lineage of
  eukaryotes
• Distinguished by segmented bodies
  and sophisticated exoskeletons
• The body is organized into distinct
  head and trunk regions
• Most species have compound eyes
• Long tentacle-like appendages called
  antennae
   Arthropoda—Myriapods
   (Millipedes, Centipedes)

• Relatively simple
  bodies with a
  series of short
  segments, each
  with one or two
  pairs of legs
Arthropoda—Chelicerata
 (Spiders, Ticks, Mites,
Horseshoe Crabs, Daddy
  Longlegs, Scorpions)
           Chelicerates
• Most are terrestrial
• Includes the arachnids
• Bodies consist of
  anterior and posterior
  regions
• Named for
  appendages called
  chelicerae found near
  the mouth
     Arthropoda—Insecta
           (Insects)

• Distinguished by having three body
  regions: head, thorax, and abdomen
• Three pairs of walking legs on the
  ventral side of the thorax
• In most species, two pairs of wings
  are mounted on the dorsal side of the
  thorax
Arthropoda—Crustaceans
 (Shrimp, Lobster, Crabs,
   Barnacles, Isopods,
       Copepods)
            Crustaceans
• Segmented body divided into the
  cephalothorax and the abdomen.
• Many crustaceans have a carapace—
  a platelike section of their exoskeleton
  that covers and protects the
  cephalothorax
• Mouthparts called mandibles that bite
  or chew.