Overview of Animal Diversity

General Features of Animals Animals are a diverse group of consumers that share major characteristics All are heterotrophs All are multicellular Cells do not have cell walls Most are able to move All are very diverse in form and habitat Most reproduce sexually Have a characteristic patterns of embryonic development Cells of all animals (except sponges) are organized into tissues

Evolution of the Animal Body Plan Five key transitions in animal evolution Tissues Symmetry Body cavity Development Segmentation

Evolution of the Animal Body Plan 1. Evolution of tissues Parazoa (Sponges - the simplest animals) lack defined tissues and organs Have the ability to disaggregate and aggregate their cells Eumetazoa (all other animals) have distinct and well-defined tissues Have irreversible differentiation for most cell types

Evolution of the Animal Body Plan 2. Evolution of symmetry Parazoa (sponges) lack any definite symmetry Eumetazoa have a symmetry defined along imaginary axes drawn through the animal’s body There are three main types of symmetry Radial symmetry Bilateral symmetry Asymmetry

Evolution of the Animal Body Plan Radial symmetry Body parts arranged around central axis Can be bisected into two equal halves in any 2D plane perpendicular to that axis

Evolution of the Animal Body Plan Bilateral symmetry Body has right and left halves that are mirror images Body has distinct anterior/posterior and dorsal/ventral divisions

Evolution of the Animal Body Plan Bilaterally symmetrical animals have two main advantages over radially symmetrical animals Cephalization - Evolution of a definite head/brain area Greater mobility

Evolution of the Animal Body Plan Asymmetry Body without symmetry Body has no distinct halves or segments that can be made that would be identical

Evolution of the Animal Body Plan 3. Evolution of a body cavity Eumetazoa produce three germ layers Outer ectoderm (body coverings and nervous system) – nervous system is different due to the way it forms Middle mesoderm (skeleton and muscles) – completely enclosed in the body Inner endoderm (digestive organs and intestines) – open to the outside via tracts

Evolution of the Animal Body Plan 3. Evolution of a body cavity Three basic kinds of body plans Acoelomates have no body cavity Ex. Flatworms (Planeria)

Evolution of the Animal Body Plan Pseudocoelomates have a body cavity between mesoderm and endoderm Called the pseudocoel Ex. Roundworms (Nematode)

Evolution of the Animal Body Plan Coelomates have a body cavity entirely within the mesoderm Called the coelom Ex. Segmented Worms (Earthworms)

Evolution of the Animal Body Plan The body cavity made possible the development of advanced organs systems Coelomates developed a circulatory system to flow nutrients and remove wastes Open circulatory system: blood passes from vessels into sinuses, mixes with body fluids and reenters the vessels Closed circulatory system: blood moves continuously through vessels that are separated from body fluids

Evolution of the Animal Body Plan 4. Evolution of different patterns of development The basic bilaterian pattern of development Mitotic cell divisions of the egg form a hollow ball of cells, called the blastula Blastula indents to form a 2-layer-thick ball called a gastrula with: Blastopore - Opening to outside Archenteron - Primitive body cavity

Evolution of the Animal Body Plan

Evolution of the Animal Body Plan Bilaterians can be divided into two groups Protostomes develop the mouth first from the blastopore Anus (if present) develops either from blastopore or another region of embryo Deuterostomes develop the anus first from the blastopore Mouth develops later from another region of the embryo

Evolution of the Animal Body Plan Deuterostomes differ from protostomes in three other embryological features: Cleavage pattern of embryonic cells Protostomes - Spiral cleavage Deuterostomes - Radial cleavage Developmental fate of cells Protostomes – Development determined; the removal of one cell stops development Deuterostomes – Development indetermined; the removal of one cell does nothing to development; the removed cell can create an additional organism Origination of coelom Protostomes - Forms simply and directly from the mesoderm; the mesoderm forms near the mouth and splits to form the coelom Deuterostomes - Forms indirectly from the archenteron; the mesodermal pouches form from the endodermal lining which eventually forms the coelom

Evolution of the Animal Body Plan

Protostomes vs. Deuterostomes The mouth develops from the first opening in the gastrula. Removal of one cell stops embryonic development. 8-cell stage has cells that are shaped like a spiral Mesoderm forms and splits down the middle to eventually form the coelom. The anus develops from the first opening in the gastrula and the mouth develops later. Removal of one cell does not stop embryonic development. 8-cell stage has cells that are directly aligned. Mesoderm forms pouches from the endoderm to eventually form coelom.

Evolution of the Animal Body Plan 5. Evolution of segmentation Segmentation provides two advantages 1. Allows redundant organ systems in adults such as occurs in the annelids 2. Allows for more efficient and flexible movement because each segment can move independently Segmentation appeared several times in the evolution of animals

DEVELOPMENT OF TISSUES Eventually the germ layers (ectoderm, endoderm, and mesoderm) begin to specialize to form tissues. All of the body tissues of animals are comprised of one of the four basic tissue types. Epithelial tissue Connective tissue Muscular tissue Nervous tissue 22

Evolutionary Developmental Biology Most taxonomists agree that the animal kingdom is monophyletic – descending from one ancestor or group of organisms (clade). Three prominent hypotheses have been proposed for the origin of metazoans from single-celled protists.

Evolutionary Developmental Biology 1. The multinucleate hypothesis – animals descended from ciliated protists but are closely related to algae 2. The colonial flagellate hypothesis - animals are descended from an ancestor that resembled a hollow spherical colony of flagellated cells called choanoflagellates 3. The polyphyletic origin hypothesis – animals descended from more than one ancestor or clade Molecular systematics using rRNA sequences settles this argument in favor of the colonial flagellate hypothesis