Chapter 3: Animal Architecture

]]Animal Body Plans]]

@@Symmetry@@

Balanced proportions – correspondence in size and shape of parts on opposite sides of a median plane

  • Spherical Symmetry:
    • Any plane passing through the center divides the body into equivalent, or mirrored, halves
    • Occurs chiefly among some unicellular eukaryote groups
    • Rare in animals
    • Best suited for floating and rolling
  • Radial Symmetry:
    • Body can be divided into similar halves by more than 2 planes passing through the longitudinal axis
    • One end of the longitudinal axis is usually the mouth
  • Biradial Symmetry:
    • Only 1 or 2 planes passing through the longitudinal axis produce mirrored halves
    • Single or paired parts limit symmetrical planes
    • E.g., ctenophores
  • Bilateral Symmetry:
    • Applies to animals divided along a sagittal plane into 2 mirrored portions
    • Right and left halves
    • Major innovation
    • Much better fitted for directional movement
    • Strongly associated with cephalization:
      • Differentiation of a head end
      • Accompanied by the concentration of nervous tissue and sense organs

@@Asymmetry@@

  • Not balanced
  • No plane through which they are divided into identical halves

]]Development of Animal Body Plans]]

An animal’s body plan forms through an inherited developmental sequence.

  • Begins with a zygote

    • Single cell
    • Will divide into a larger number of smaller cells

  • Blastomeres

  • Process = cleavage

    • Orderly sequence of cell divisions
    • Occurs in several different ways

  • Sponges and cnidarians lack a distinct pattern

  • Bilaterians typically exhibit 2 types:

    1. @@Radial Cleavage@@

      1. Tiers or layers of cells on top of one another
      2. Typically occurs with regulative development
      • If blastomere is separated from others, can adjust or “regulate” its development
      • Result: complete embryo

    2. @@Spiral Cleavage@@

      1. Cleavage planes diagonal to the polar axis
      2. Unequal cells are produced by alternate clockwise and counterclockwise cleavage around the axis of polarity
      3. Typically occurs with a form of mosaic development

      • Organ-forming determinants in egg cytoplasm are positioned within egg

        • Before first cleavage division
        • Result: separated blastomeres still develop as if part of the whole
        • Defective, partial embryo

        radial vs. spiral cleavage

@@Cleavage@@

  • Proceeds until the zygote is divided into many small cells

    • Surround a blastocoel: Fluid-filled cavity.

  • Blastula

    • Hollow ball of cells
    • Becomes a gastrula
    • Except in sponges
    • Process: gastrulation

    development of an animal

@@Gastrulation@@

  • 2-3 germ layers develop
    • Primary cell layers
    • Some of the first lineage-specific stem cells
    • Ectoderm
    • Outer layer
    • Differentiates into epidermis and nervous system cells
    • Endoderm
    • Innermost layer
    • Surrounds and defines inner body cavity = Gastrocoel
      • Will become gut cavity
    • Mesoderm
    • Middle germ layer
    • Gives rise to connective tissues, muscle, urogenital and vascular systems, and peritoneum

@@Body Cavities@@

  • Gut cavity

    • Development from gastrocoel
    • Always has at least 1 opening
    • Blastophore

  • One opening: blind or incomplete gut

  • Most animals develop 2nd opening to gut

    • Creates a tube
    • Complete gut
    • Typically surrounded by a fluid-filled cavity
    • Coelom if lined with mesoderm

  • Coelom

    • Body cavity in triploblastic animals
    • Lined with mesodermal peritoneum
    • In some animals, mesoderm only lines the outer edge of the blastocoel
    • Lies next to ectoderm
    • Pseudocoelom
    • In some animals, mesoderm completely fills blastocoel
    • Acoelomate animals

    development

  • Evolution was a major development for bilaterians

  • Advantages:

    • Tube-within-a-tube
    • Space for viscera, cushioning, protection, hydrostatic skeleton (aids movement)

]]How Many Body Plans Are There?]]

Animals comprise 32 phyla.

  • Sponges only have a cellular level of organization

  • All others:

    • Diploblastic: 2 germ layers. (E.g., Phylum Cnidaria)
    • Triploblastic: 3 germ layers
    • Bilateria: 2 groups that differ in various developmental characteristics
      • Protostomia
      • Deuterostomia

    Protostomia and Deuterostomia

@@Deuterostome Body Plans:@@

  • Blastopore becomes anus

    • “Second mouth”
    • Refers to formation of mouth from a second opening in embryo

  • 3 Deuterostome phyla:

    1. Echinodermata (Sea stars and relatives)

    2. Hemichordata

    3. Chordata

    Hemichordata

    Chordata

@@Protostome Body Plans:@@

  • First embryonic opening (blastopore) becomes the mouth
  • 2 subgroups:
    • Ecdysozoa
    • Molting animals
    • Arthropods, nematodes, and 6 other phyla
    • Lophotrochozoa
    • Very diverse group
    • 17 phyla

]]Components of Animal Bodies]]

All animal bodies consist of:

  1. @@Cellular Components@@
  • Tissues and organs derived from embryonic germ layers
    • A group of cells specialized for performing a common function
  • Study of tissues = histology
  • 4 kinds:
    • Epithelial
    • Connective
    • Muscular
    • Nervous
  1. @@Extracellular Components@@
    • Fluids and structures that cells deposit outside their cell membranes

@@Epithelial Tissue@@

  • Sheets of cells
  • Cover external or internal surfaces
  • Often modified into glands that produce mucus, hormones, or enzymes

@@Connective Tissue@@

  • Diverse group
  • Various binding and supportive functions
  • Widespread throughout body

@@Muscle Tissue@@

  • Most common tissue of most animals
  • Originates from mesoderm
  • Specialized for contraction
  • Types: Skeletal, Cardiac, Smooth

@@Nervous Tissue@@

  • Specialized for receiving stimuli and conducting impulses from one region to another

  • Types of cells:

    • Neurons
    • Neuroglia

    Types of Tissues