Introduction to AnimalsIntroduction to Animals

Development, Symmetry, Tissues, Classification

AP BiologyUnit 6

Defining Characteristics of Animals

• Multicellular

• Heterotrophs

• Sexual reproduction followed by development of body tissues and organs from layers of embryonic tissue

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Animal Development

• Following fertilization, animal embryos develop in a systematic fashion

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Animal Development• Cleavage

– A series of rapid cell divisions – No time for cells to grow in between divisions

• Gastrulation – Formation of layers of tissue that eventually

develop into body parts and organs

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Body Symmetry

• 3 different categories of body symmetry– No symmetry (Asymmetrical)

• Ex. Sponges (Porifera)– Radial symmetry– Bilateral symmetry

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Radial Symmetry

• One main axis in the center

• No left or right sides– dividing the animal along any axis makes it a mirror image on each side.

• Ex. Sea Anemones, Cnidarians (jellyfish)

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Bilateral Symmetry

• Has a left and right side– only one plane will divide the animal so that the two sides are mirror images of one another.

• Ex. Humans, fish, insects (most animals have bilateral symmetry)

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Directional Terms

• There are certain directional terms that can be associated with bilateral symmetry

• Dorsal = top side (think dorsal fin on dolphins)

• Ventral = bottom side

• Anterior = the end towards the head

• Posterior = the end towards the tail

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Embryonic Germ Layers

• Adult body parts develop from embryonic germ layers

• There are 3 different germ layers– Ectoderm– Mesoderm– Endoderm

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Ectoderm

• Outermost layer

• Develops into skin, nervous system, sensory organs

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Mesoderm

• Middle layer

• Develops into muscles, bones, circulatory system, excretory system, most of reproductive system

• Not all animals have mesoderm

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Endoderm

• Inner most layer

• Develops into the digestive system, respiratory system

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Diplo or triplo?

• Not all animals have all 3 germ layers• Diploblastic = only have 2 germ layers (no

mesoderm, only endoderm and ectoderm)– Ex. Cnidarians (jelly fish)

• Triploblastic = have all 3 germ layers – All bilaterally symmetrical animals (worms,

birds, mammals, etc)– Closely linked to cephalization (concentration

of nervous system and sensory organs close to head region)

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Body Cavities

• Most animals have a body cavity (fluid filled space) between the endoderm and ectoderm

• Coelom = body cavity lined with mesoderm

• Type of body cavity influences how the animal moves

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Types of Body Cavities

• Acoelomate– Lack an enclosed body cavity– Instead, the space is filled with cells– Ex. Flatworm

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Types of Body Cavities

• Pseudocoelomate– Have a fluid filled space

in which many organs are suspended

– Only one side (outer side) is lined with mesoderm (muscle)

– Can’t control body shape as well since only one layer of muscle

– Ex. Roundworm

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Types of Body Cavities

• Coelomate– Have a coelom – body cavity lined with

mesoderm on both sides– Better control over movement since there are

more muscles present (usually going in several different directions)

– Ex. Earthworm, humans

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Protostomes vs. Deuterostomes

• Excluding sponges, cnidarians and ctenophores, all other animals can be classified as either a protostome or a deuterostome.

• Classification has to do with how the embryo develops.

• Early in development a cavity forms

• Blastopore = opening to the cavity

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Protostomes

• Blastopore becomes the mouth of the animal (means “mouth first” in Greek)

• Cleavage– Spiral = cleavage occurs in planes

diagonal to the vertical– Determinate = The “fate” of each

cell is determined very early – Examples include: molluscs,

annelids (earthworms), arthropods

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Deuterostomes• Blastopore becomes the anus

(means “mouth second” in Greek)• Cleavage

– Radial = cleavage occurs along lines that are parallel or perpendicular to the vertical

– Indeterminate = the “fate” of each cell is not determined until a little later (early cells can still become any type of cell– “stem cells”)

– Examples include: Echinoderms, Chordates (have a backbone)

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Animal Classification

• Based on developmental morphology

• Molecular evidence suggests that this classification may not be entirely accurate

• Common ancestor of all animals is thought to be a colonial protist with a flagella

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Animal Classification

• “Basal” animal = porifera (sponge) = simplest animal; at the bottom of the evolutionary tree

• Most animals have bilateral symmetry

• All animals are thought to belong to a group called Metazoa

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Scientific Names for Animals

• Porifera = sponges• Cnidarians = jellyfish• Platyhelminthes = flatworms• Nematodes = roundworms • Annelids = Earthworms• Arthropods = insects, crustaceans• Echinoderms = starfish• Chordates = humans, birds, fish, amphibians

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