Animal Origins and the Evolution of Body Plans

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Animal Origins and the Evolution of Body Plans
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Animal Origins and the Evolution of Body Plans

• Animals Descendants of a Common Ancestor
• Body Plans Basic Structural Designs
• Sponges Loosely Organized Animals
• Cnidarians Two Cell Layers and Blind Guts
• Ctenophores Complete Guts and Tentacles
• The Evolution of Bilaterally Symmetrical Animals
• Simple Lophotrochozoans
• Lophophorates An Ancient Body Plan
• Spiralians Spiral Cleavage and Wormlike Body
  Plans

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Animals Descendants of a Common Ancestor

• Evidence indicates that all animals are
  descendants of a single ancestral lineage.
• All animals share a set of derived traits
• Similarities in their small-subunit ribosomal
  RNAs
• Similarities in their Hox genes
• Special types of cellcell junctions tight
  junctions, desmosomes, and gap junctions
• A common set of extracellular matrix molecules,
  including collagen

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Animals Descendants of a Common Ancestor

• Animals evolved from ancestral colonial
  flagellated protists.
• Within these ancestral colonies, a division of
  labor arose.
• Cells became specialized for different functions,
  such as movement, nutrition, and reproduction.
• The specialized units continued to differentiate
  while improving their coordination with other
  working groups of cells.
• These coordinated groups of cells evolved into
  animals.

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Animals Descendants of a Common Ancestor

• Generalized traits characterize animals
• They are multicellular organisms that must take
  in pre-formed organic molecules.
• They acquire these organic molecules by ingesting
  other organisms, living or dead, and digesting
  them within their bodies.
• Animals must expend energy to acquire these
  organic molecules.
• Most have circulatory systems that carry O2, CO2,
  and nutrients.

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Animals Descendants of a Common Ancestor

• Much of the diversity in the animal kingdom
  evolved as animals acquired the ability to
  capture and eat many different types of food and
  to avoid becoming food for other animals.
• The need to move in search of food has favored
  sensory structures that provide animals with
  detailed information about their environment.
• Animals expend a considerable amount of energy to
  maintain relatively constant internal conditions
  while taking in foods that vary chemically.

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Animals Descendants of a Common Ancestor

• Clues to the evolutionary relationships among
  animals are found in the fossil record, patterns
  of embryonic development, comparative physiology
  and morphology, and the structure of molecules
  such as the small-subunit RNAs and mitochondrial
  genes.
• The sponges, cnidarians, and ctenophores
  separated from the other animal lineages early in
  evolutionary history.
• The remaining animals have been divided into two
  major lineages the protostomes and the
  deuterostomes.

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Figure 32.1 A Current Phylogeny of the Animals
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Animals Descendants of a Common Ancestor

• Animals form layers of cells during their
  development from a single-celled zygote to a
  multicellular adult.
• The embryos of diploblastic animals have two cell
  layers an outer ectoderm and an inner endoderm.
• The embryos of triploblastic animals have a third
  layer, the mesoderm.
• The existence of three cell layers distinguishes
  the protostomes and deuterostomes from simple
  animals that diverged earlier.

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Animals Descendants of a Common Ancestor

• Protostomes and deuterostomes differ in the fate
  of the blastopore , the opening of the cavity
  that forms in the spherical embryo.
• In the protostomes, the mouth arises from the
  blastopore.
• In the deuterostomes, the blastopore gives rise
  to the anus.

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Animals Descendants of a Common Ancestor

• Most protostomes and the deuterostomes exhibit a
  pattern of early cell division in the fertilized
  egg called radial cleavage.
• In radial cleavage, cells divide along a plane
  either parallel to or at right angles to the long
  axis of the fertilized egg.
• One major protostome lineage evolved a pattern of
  early cell division called spiral cleavage.

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Body Plans Basic Structural Designs

• The entire structure of an animal, its organ
  systems, and the integrated functioning of its
  parts are known as its body plan.

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Body Plans Basic Structural Designs

• Overall shape is referred to as symmetry. A
  symmetrical animal can be divided into similar
  halves along at least one plane.
• Animals that have no plane of symmetry are said
  to be asymmetrical.
• In spherical symmetry body parts radiate out from
  a central point. Spherical symmetry is widespread
  among the protists.
• An organism with radial symmetry has one main
  axis around which its body parts are arranged.
• Bilaterally symmetric animals can be divided into
  mirror images by a single plane.

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Figure 32.2 Body Symmetry
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Body Plans Basic Structural Designs

• Bilateral symmetry is a common characteristic of
  animals that move freely through their
  environments.
• Bilateral symmetry is often associated with
  cephalization the presence of a head bearing
  sensory organs and central nervous tissues at the
  anterior end of the animal.

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Body Plans Basic Structural Designs

• Body cavities are fluid-filled spaces that lie
  between the cell layers of the bodies of many
  kinds of animals.
• The type of body cavity an animal has influences
  how it can move.
• Animals can be grouped into three major
  categories based on the type of body cavity they
  have the acoelomates, the pseudocoelomates, and
  the coelomates.

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Body Plans Basic Structural Designs

• Acoelomates lack an enclosed body cavity. The
  space between the gut and body wall is filled
  with cells called mesenchyme.
• Pseudocoelomates have a pseudocoel, a liquid
  filled space in which organs are suspended.
• Coelomates have a coelom that develops within the
  mesoderm. It is lined with the peritoneum and
  enclosed on the inside and outside by muscles.

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Figure 32.3 Animal Body Cavities (Part 1)
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Figure 32.3 Animal Body Cavities (Part 2)
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Body Plans Basic Structural Designs

• The fluid-filled body cavities of simple animals
  function as hydrostatic skeletons.
• When the muscles surrounding fluids contract, the
  fluids can be moved to other parts of the body,
  causing these body regions to expand.
• Other forms of skeletons developed in many
  lineages, including internal skeletons
  (vertebrate bones), and external skeletons (crab
  shells, clam shells).
• The form of an animals skeleton and body
  cavities strongly influences the degree to which
  it can control and change its shape and thus the
  complexity of the movements it can perform.

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Sponges Loosely Organized Animals

• The lineage leading to modern sponges (phylum
  Porifera) separated from the lineage leading to
  other animals very early during animal evolution.
• Sponges are sessilethey live attached to the
  substratum.
• The body plan of sponges is an aggregation of
  cells built around a water canal system.

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Figure 32.4 The Body Plan of a Simple Sponge
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Sponges Loosely Organized Animals

• Sponges have a supporting skeleton, either in the
  form of branching spines called spicules or as an
  elastic network of fibers.
• Sponges are loosely organized if a sponge is
  completely disassociated, its cells can
  reassemble into a new sponge.
• Sponges depend on water movement through their
  bodies to obtain food and are often oriented at
  right angles to current flow so that they may
  intercept water as it flows past.
• Sponges reproduce both sexually and asexually. In
  most species, a single individual produces both
  eggs and sperm. Asexual reproduction is by
  budding and fragmentation.

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Figure 32.5 Sponges Differ in Size and Shape
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Cnidarians Two Cell Layers and Blind Guts

• The cnidarians (phylum Cnidaria) were the next
  lineage to split off from the main line of animal
  evolution after the sponges.
• They are diploblastic and have a blind gut with
  only one entrance.
• Despite their relatively simple structures, the
  Cnidarians have structural molecules, such as
  actin and collagen, and homeobox genes.

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Cnidarians Two Cell Layers and Blind Guts

• Cnidarians appeared early in evolutionary history
  and radiated in the late Precambrian.
• There are about 11,000 species living today.
• The cnidarian body plan combines a low metabolic
  rate with the ability to capture large prey,
  allowing cnidarians to survive in environments
  where prey is scarce.

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Cnidarians Two Cell Layers and Blind Guts

• Cnidarians have tentacles with specialized cells
  called cnidocytes. These cells contain structures
  called nematocysts that can discharge toxins into
  their prey.
• The mouth of a cnidarian is connected to a blind
  sac called the gastrovascular cavity. It
  functions in digestion, circulation, and gas
  exchange.
• Cnidarians have epithelial cells with muscle
  fibers whose contractions allow them to move, as
  well as nerve nets that integrate body activities.

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Figure 32.7 Nematocysts Are Potent Weapons
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Cnidarians Two Cell Layers and Blind Guts

• The generalized cnidarian life cycle has two
  stages
• The polyp is typically asexual individual polyps
  may reproduce by budding to form colonies.
• The medusae produce eggs and sperm and release
  them into the water.
• A fertilized egg becomes a free-swimming,
  ciliated larva called a planula that eventually
  settles to the bottom and transforms into a polyp.

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Figure 32.8 A Generalized Cnidarian Life Cycle
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Cnidarians Two Cell Layers and Blind Guts

• Corals are also usually sessile and colonial.
• The polyps of corals secrete a matrix of organic
  molecules upon which calcium carbonate is
  deposited.
• This matrix forms the eventual skeleton of the
  coral colony.
• As coral colonies grow, old polyps die and leave
  their calcareous skeletons behind.
• Living members of the colony form a layer on top
  of a growing reef of skeletal remains.

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Figure 32.9 Corals (Part 1)
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Figure 32.9 Corals (Part 2)
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The Evolution of Bilaterally Symmetrical Animals

• A common ancestor of all bilaterally symmetrical
  animals is postulated.
• Zoologists use evidence from genes, development,
  and the structure of existing animals to infer
  the form of ancient bilaterians.
• The development of all bilaterally symmetrical
  animals is controlled by homologous Hox and
  homeobox genes. It is unlikely that these genes
  evolved separately in several animal lineages.
• Fossilized tracks from the late Precambrian
  suggest that early bilaterians had circulatory
  systems, antagonistic muscles, and a tissue- or
  fluid-filled body cavity.

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Figure 32.13 The Trail of an Early Bilaterian
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The Evolution of Bilaterally Symmetrical Animals

• The protostomes and the deuterostomes that
  dominate todays fauna have been evolving
  separately since the Cambrian period.
• Members of both lineages are bilaterally
  symmetrical and have cephalization.

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The Evolution of Bilaterally Symmetrical Animals

• Shared, derived traits that unite the protostomes
  include
• A central nervous system consisting of an
  anterior brain that surrounds the entrance to the
  digestive tract
• A ventral nervous system consisting of paired or
  fused longitudinal nerve cords
• Free-floating larvae with a food-collecting
  system consisting of compound cilia on
  multiciliate cells
• A blastopore that becomes the mouth
• Spiral cleavage (in some species)

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The Evolution of Bilaterally Symmetrical Animals

• The major shared, derived traits that unite the
  deuterostomes inlcude
• A dorsal nervous system
• Larvae, if present, that have a food-collecting
  system consisting of cells with a single cilium
• A blastopore that becomes the anus
• Radial cleavage

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Simple Lophotrochozoans

• The flatworms (phylum Platyhelminthes) are the
  simplest of the lophotrochozoans.
• The flatworms are bilaterally symmetrical,
  unsegmented, acoelomate animals.
• They lack organs for transporting oxygen to
  internal tissues.
• They have simple organs for excreting metabolic
  wastes.
• Their flattened form allows each body cell to be
  near a body surface, a requirement of their body
  plan.

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Simple Lophotrochozoans

• The flatworm digestive tract is a mouth opening
  into a blind sac.
• The sac is often highly branched, increasing the
  surface area available for the absorption of
  nutrients.
• Flatworms feed on living or dead animal tissue.
• The motile flatworms move by beating broad bands
  of cilia.

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Simple Lophotrochozoans

• The flatworms of the class Turbellaria are
  probably most similar to ancestral flatworm
  forms.
• Turbellarians are small, free-living, marine and
  freshwater animals.
• The head has chemoreceptor organs, simple eyes,
  and a small brain.

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Figure 32.15 Flatworms Live Freely and
Parasitically (Part 1)
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Simple Lophotrochozoans

• Most living flatworms are parasitic, such as the
  tapeworms (class Cestoda) and the flukes (class
  Trematoda).
• Parasitic flatworms lack digestive tracts they
  absorb digested food from their hosts.
• Some species cause serious diseases, such as
  schistosomiasis.
• Most parasitic species have complex life cycles
  involving one or more intermediate hosts and
  several larval stages.

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Figure 32.15 Flatworms Live Freely and
Parasitically (Part 2)
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Figure 32.16 Reaching a Host by a Complex Route
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Lophophorates An Ancient Body Plan

• The brachiopods (phylum Brachiopoda) are
  solitary, marine lophophorate animals that
  superficially resemble bivalve mollusks.
• The shell differs from that of mollusks in that
  its two halves are dorsal and ventral rather than
  lateral.
• Brachiopods are either attached to a solid
  substrate by a short, flexible stalk or embedded
  in soft sediment.
• Most species release gametes into the water,
  where they are fertilized.
• More than 26,000 fossil species have been
  described, but only 350 species survive today.

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Figure 32.20 Brachiopods
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Spiralians Spiral Cleavage and Wormlike Body
Plans

• Ribbon worms (phylum Nemertea) are carnivorous
  spiralians.
• They are similar in structure to the flatworms,
  but they have a complete digestive tract.
• Small ribbon worms move by beating their cilia
  larger ones move by waves of contraction of body
  muscles.

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Spiralians Spiral Cleavage and Wormlike Body
Plans

• A body cavity that is segmented allows an animal
  to alter the shape of its body in complex ways
  and to control its movements precisely.
• Segmentation evolved several times among
  spiralians.
• The annelids (phylum Annelida) are a diverse
  group of segmented worms.
• Annelid species can be found in marine,
  freshwater, and terrestrial environments.

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Spiralians Spiral Cleavage and Wormlike Body
Plans

• Nerve cord is found on the ventral side.
• Each segment in an annelid is controlled by a
  separate nerve center called a segmented
  ganglion. All the ganglia are connected by nerve
  cords that coordinate their function.
• The coelom in each segment is isolated from those
  in other segments.
• Most species lack a rigid, external protective
  surface.
• The thin body wall serves as a surface for gas
  exchange and also limits annelids to moist
  environments, as they lose body water rapidly in
  dry air.

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Figure 32.22 Annelids Have Many Body Segments
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Spiralians Spiral Cleavage and Wormlike Body
Plans

• The mollusks (phylum Mollusca) range in size from
  small snails to giant squids that can be more
  than 18 meters long.
• Mollusks have a unique body plan with three major
  structural components foot, mantle ( a hard
  skeleton structure), and visceral mass that
  covers the internal organs.
• The molluscan foot is a large, muscular structure
  that originally was both an organ of locomotion
  and support for the internal organs.

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Figure 32.25 Molluscan Body Plans (Part 1)
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Spiralians Spiral Cleavage and Wormlike Body
Plans

• The bivalves (class Bivalvia) have a hinged,
  two-part shell that extends over the sides and
  top of their body.
• Bivalves are largely sedentary.
• They have greatly reduced heads.
• Feeding is accomplished by bringing water in
  through an opening called an incurrent siphon and
  extracting food from the water using their gills.
• Water and gametes exit through another opening,
  the excurrent siphon.

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Figure 32.26 Diversity among the Mollusks (Part
2)
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Spiralians Spiral Cleavage and Wormlike Body
Plans

• The gastropods (class Gastropoda) are mostly
  motile, using their large foot to move across a
  substrate or to burrow through it.
• The gastropods are the most species-rich and
  widely distributed of the molluscan classes.
• Some gastropods can crawl, whereas others have a
  modified foot that functions as a swimming organ.
• Gastropods are the only terrestrial mollusks.
  They have a mantle cavity that is modified into a
  highly vascularized lung.

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Figure 32.25 Molluscan Body Plans (Part 4)
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Spiralians Spiral Cleavage and Wormlike Body
Plans

• The cephalopods (class Cephalopoda) have a
  modified excurrent siphon.
• This modification allowed early cephalopods to
  control the water content of the mantle cavity.
• The modification of the mantle into a device for
  forcibly ejecting water from the cavity enabled
  cephalopods to move rapidly through the water.
• It also allows the animals to control their
  buoyancy.
• Their greatly enhanced mobility allowed some
  cephalopods, such as squids and octopuses, to
  become the major predators in open ocean waters.

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Figure 32.25 Molluscan Body Plans (Part 5)
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Figure 32.26 Diversity among the Mollusks (Part
4)
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Spiralians Spiral Cleavage and Wormlike Body
Plans

• Cephalopods include the squids, octopuses, and
  nautiluses.
• They appeared near the beginning of the Cambrian
  period about 600 million years ago.
• They were the first large, shelled animals able
  to move vertically in the ocean.
• Nautiloids are the only cephalopods with external
  chambered shells that survive today.

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Figure 32.26 Diversity among the Mollusks (Part
5)