Chapter 32

An Introduction to Animal Diversity

Modes of Nutrition

• Animals differ in their mode of nutrition than plants and fungi.– Animals and fungi are heterotrophic.– Plants are autotrophic.– Fungi release exoenzymes.

• Animals ingest their food.– They release enzymes to break down

their food.

Animal Cells

• Animal cells lack cell walls.• Fungi have them--• Plants have them--• The structural integrity of animal

bodies is due to proteins--collagen.– Tight junctions, gap junctions, and

desmosomes.

• Muscle cells and nerve cells are also found in animals.

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Animal Reproduction

• Animals reproduce mostly via sexual reproduction.– Life cycle dominated by the diploid

stage.

• Usually a small, haploid, flagellated sperm fertilizes a non-motile egg forming a diploid zygote.

Cell Division

• Cell Division is known as cleavage

Changes in the Zygote

• After fertilization, the zygote undergoes cleavage.

• Successive mitotic divisions with no cell growth.

• This division leads to the formation of the blastula.

The Blastula

• Is a hollow ball of animal cells.• The blastula then becomes the

gastrula.

The Gastrula

• The gastrula is the stage that gives rise to embryonic tissues.

• The embryonic tissues eventually develop into adult body parts.

Stages of Development

• Some animals go through transient stages of development.– Many of them go through at least 1

larval stage.

• A larva is a sexually immature form of an animal that is morphologically distinct from the adult.– Frogs and flies are examples.

Stages of Development

• Recall the importance of the homeobox genes.– Hox genes control the segmentation pattern in

animals.

• They are the result of many successive gene duplications throughout evolution.

• They play important roles in the development and differentiation of animals.

• They produce many of the observed morphological features.

Body Plans

• Body plans are morphological traits or organizational plans that are shared by a group of animal species.

Symmetry

• Some animals have symmetry, others don’t.

• Radial symmetry--top and bottom, but no front and back.

• Bilateral symmetry--top and bottom, front and back.

Tissues

• The body plans of animals also varies according to tissue organization.

• Tissues are collections of specialized cells separated from other tissues by membranous layers.

• Tissues form after gastrulation has taken place.

Tissues

• Development gives rise to germ layers which form the various tissues and organs of the body.– Ectoderm--outer portion of body--skin.– Endoderm--innermost layer. Lines the

developing digestive tube. The lining of the digestive tract and organs derived from it.

Germ Layers

• Ectoderm and endoderm are the previously mentioned germ layers.

• Animals with just these two are called diploblasts.– Sponges and Coelenterates

• Animals that have a third germ layer, the mesoderm, are called triploblasts.– Mesoderm forms the muscles and most other

organs between the digestive tube and the outside of the animal.

Body Cavities

• Some triploblasts have a body cavity filled with fluid.– Called a coelom; derived from

mesoderm.

• It separates the digestive tract from the outer body wall.– Animals with a coelom are called

coelomates.

Body Cavities

• Some triploblasts form a body cavity from the blastocoel rather than the mesoderm.– This cavity is functional and is called a

pseudocoelom.– These animals are called

pseudocoelomates.

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Body Cavities

• Some triploblasts lack a coelom altogether.– There is no body cavity between the

digestive tract and the outer body wall.– These are called acoelomates.

Protostomes and Deuterostomes

• Many animals can be classified as either protostomes or deuterostomes.

• Three features distinguish their development:– 1. Cleavage– 2. Coelom Formation– 3. Fate of the Blastopore

Cleavage• Many protostomes have spiral cleavage.• Cleavage diagonal to the vertical axis of the

embryo.• The 8-cell stage has smaller cells which lie in the

grooves between the larger underlying cells.

Cleavage• This so-called determinant cleavage

of some animals determines the developmental fate of each embryonic cell very early.

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Cleavage

• Radial cleavage is common in deuterostomes.

• You either see cleavage planes parallel or perpendicular to the vertical axis of the embryo.

• Here, the tiers of the cells are aligned.

Cleavage

• Most deuterostomes have indeterminant cleavage.

• This means that each cell in the embryo can fully develop into a complete embryo.– This makes identical twins possible.– This is why 4-cell stage sea stars can be divided

to give 4 larvae.– This is why embryonic stem cells have the

capacity to perform so many functions.

Deuterostomes• Have radial cleavage• Their archenteron gives rise to the coelom• The mouth arises from the end of the

embryo opposite the blastopore.• (The blastopore becomes the anus)

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The Archenteron• This is the endoderm-lined cavity

that forms during animal development (gastrulation) and gives rise to the digestive tract.

Coelom Formation

• In protostome development, as the archenteron forms during gastrulation, the mesoderm splits forming a coelomic cavity.

Coelom Formation

• During deuterostome development, the mesoderm buds off of the wall of the archenteron.

• This cavity becomes the coelom.

Fate of the Blastopore

• This is the fundamental characteristic that distinguishes protostome and deuterostome development.

• The blastopore is the indentation that leads to the formation of the archenteron.

Protostomes

• After the archenteron develops, a second opening forms at the opposite end of the gastrula.

• Now there are two openings.

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Protostomes

• In protostomes, the mouth develops from the first opening--the blastopore.

• The anus develops from the second opening.

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Deuterostomes

• In deuterostomes, the mouth develops from the second opening.

• The blastopore (first opening) becomes the anus.

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