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

        

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

  

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

  

• 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

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

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

2. 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