Overview of Animal Diversity

Definition of Animals:
- Multicellular
- Heterotrophic
- Eukaryotic
- Develop from embryonic layers
Exceptions to Living Things:
- Example: Plasmodium falciparum
- Considered by many to be an animal despite being unicellular
- Previously covered with protists
Importance of Criteria:
- Most animals fit the established criteria.

How Animals Eat:
- Animals cannot synthesize their own organic materials (unlike plants).
- Plants: autotrophic
- Fungi: obtain nutrition by decomposing organic matter
- Most animals: dissolve materials within their bodies for nutrition.

General Cell Structure:
- Most animals are eukaryotes and multicellular.
- Lack of Cell Walls:
- Unlike plants and fungi, animals do not have cell walls.
- Animals use proteins, including collagen, in their cell membranes to connect cells.
Tissue Specialization:
- Presence of specialized tissues (e.g., nervous and muscle tissues).
- Unique capabilities that fungi and plants do not possess.

Modes of Reproduction:
- Most animals reproduce sexually, with many exceptions.
Dominance of Diploid Stage:
- Diploid stage is the prevalent state in the animal lifecycle.
- Sperm and egg production occurs via meiosis.
Zygote Development:
- Zygotes are formed when small, flagellated sperm fertilizes larger, nonmotile eggs.
- Zygotes undergo mitotic divisions:
- Cleavage: series of mitotic divisions sans cell growth.
- Blastula Formation: A multicellular stage that forms a hollow ball of cells, leading to gastrulation.
Gastrulation Explained:
- The process where one end of the embryo folds inward.
- Forms multiple layers of embryonic tissues:
- Ectoderm: outer layer.
- Endoderm: inner layer.

Stages in Embryonic Development:
- Zygote undergoes cleavage to form a multicellular blastula.
- Blastula develops into a gastrula, which involves an indentation (blastopore).

Adult Development:
- Many animals develop directly into adults but others have larval stages.
Definition of Larva:
- A sexually immature form, distinct from the adult.
- Engages in different feeding behaviors from adults.
Metamorphosis:
- Developmental transformation from larva to adult.
- Some animals undergo a single larval stage, while others may have multiple.

Estimates of Animal Species:
- Approximately 1.3 million extant (living) species, with many more undiscovered.
Tremendous Diversity of Animal Types:
- Examples include corals, cockroaches, crocodiles, humans, and snails.
Common Ancestor of Animals:
- Likely lived around 770 million years ago.
- Closest related protist group: Choanoflagellates.
Ediacaran Biota:
- Members resemble various animal forms including mollusks, sponges, and cnidarians.

Timeline: 541-252 million years ago.
Cambrian Explosion:
- Major increase in animal diversity occurring between 535-525 million years ago.
- Emergence of bilaterans.
Definition of Bilaterans:
- Animals exhibiting bilateral symmetry.
- Notable exceptions: sponges and cnidarians.

Diversity Growth:
- Continued increase in animal diversity during the Ordovician, Silurian, and Devonian periods.
- Existence of mass extinctions.
Marine Dominance:
- Vertebrates were the dominant sea animals.
- Fish and arthropods (e.g., spiders, millipedes, centipedes) were the first to colonize land.

Timeline: 66 million years ago to present.
Climate and Ecological Changes:
- Extinction of non-flying dinosaurs and marine reptiles.
- Decline in climate temperature, leading to different vegetation types.
- Replacement of dense forests in Africa with savannas and open woodlands which benefited primates, leading to the evolution of human ancestors.

Definition of Body Plan:
- Morphological and developmental traits integrated into a functional whole.
Misconception of “plan”:
- Animals did not consciously design their morphology (e.g., blobfish, naked mole rat).

Symmetry Variations:
- No Symmetry: Example: sponges.
- Radial Symmetry: Predominantly in cnidarians (e.g., jellyfish, sea anemones).
- Bilateral Symmetry: Most animals characterized by two axes.
- Dorsal (top) and ventral (bottom).
- Anterior (front) and posterior (back).
Lifestyle Associations:
- Radially symmetrical animals tend to be sessile.
- Bilateral animals are usually more active.

Definition of Tissues:
- Specialized groups of cells that work together.
- Example: sponges lack true tissues; other animals develop tissues during gastrulation.
Germ Layers Defined:
- Ectoderm: outer layer of the embryo.
- Endoderm: innermost layer giving rise to organ linings.
- Diploblastic: Radially symmetrical animals possess 2 layers (ectoderm and endoderm).
- Triploblastic: Bilateral animals with three layers (ectoderm, endoderm, and mesoderm).

Body Cavities in Triploblastic Animals:
- Cavities are located between the digestive tract and body wall, called coelom.
Functionality of Body Cavity:
- Provides cushioning for suspended organs and contains fluid to shape the body.
- Facilitates organ growth and movement without disruption.

Key Differences:
- Cleavage type, coelom formation, and blastophore configuration differ.
Blastopore: The indentation leading to gastrulation.
Developmental Patterns:
- Protostomes: Develop through spiral cleavage, with determinate cleavage meaning early cell fate.
- Deuterostomes: Develop through radial cleavage and possess indeterminate cleavage allowing early cells to maintain totipotency (example: identical twins).

Archenteron: The gut-forming blind pouch produced during gastrulation.
Coelom Development:
- In protostomes: mesoderm solid masses form the coelom.
- In deuterostomes: mesoderm buds from the archenteron to form the coelom.
Forming Openings:
- In protostomes, the mouth develops first; in deuterostomes, the anus forms first.