Animal Evolution and Diversity: Study Notes

Animal Evolution and Diversity

18.1 What is an Animal?

Animals are a diverse group of eukaryotic, multicellular organisms that are heterotrophic and lack cell walls. They are primarily distinguished by their mode of nutrition, development, and genetic regulation.

• Eukaryotic, Multicellular Heterotrophs: Animals obtain nutrients by ingestion (eating other organisms or organic material).

• Cell Structure: Animal cells lack rigid cell walls, unlike plants and fungi.

• Reproduction: Most animals are diploid and reproduce sexually. Eggs and sperm are haploid cells produced by meiosis; fertilization forms a diploid zygote.

• Development: The zygote divides by mitosis to form a hollow ball of cells called a blastula. One side of the blastula folds inward, forming a gastrula with three embryonic layers:

  • Endoderm: Forms the lining of the future digestive tract.

  • Ectoderm: Forms the outer layer, giving rise to skin and nervous system.

  • Mesoderm: Forms muscles and most internal organs.

• Larval Stages and Metamorphosis: Some animals develop through larval stages, undergoing metamorphosis to become adults.

• Genetic Regulation: Homeotic genes control the transformation from zygote to adult.

Example: Sea stars develop from a larva that looks different from the adult and undergoes metamorphosis.

18.2 Animal Diversification and the Cambrian Explosion

Animal diversity increased dramatically during the Cambrian period, over 500 million years ago.

• Fossil Record: Oldest animal fossils are 575–550 million years old.

• Cambrian Explosion: Rapid diversification of animal forms occurred 535–525 million years ago.

• Burgess Shale: Famous fossil site with well-preserved Cambrian animals.

• Possible Causes:

  • Complex predator-prey relationships

  • Increase in atmospheric oxygen

18.3 Animal Body Plans

Animals are classified based on their body plans, which include symmetry, tissues, embryonic layers, body cavities, and embryonic development.

• Symmetry:

  • Radial Symmetry: Body parts arranged around a central axis (e.g., jellyfish).

  • Bilateral Symmetry: Right and left sides are mirror images; have anterior (head), posterior (tail), dorsal (back), and ventral (belly) surfaces (e.g., humans, insects).

• Tissues: Specialized collections of cells. Sponges lack true tissues; all other animals have them.

• Embryonic Layers:

  • Diploblastic: Only ectoderm and endoderm (e.g., cnidarians).

  • Triploblastic: Ectoderm, mesoderm, and endoderm (most animals).

• Body Cavities:

  • Coelom: True body cavity completely lined by mesoderm-derived tissue.

  • Pseudocoelom: Body cavity not fully lined by mesoderm (e.g., roundworms).

  • Hydrostatic Skeleton: Fluid-filled cavity provides support in soft-bodied animals.

• Embryonic Development:

  • Protostomes: Mouth develops from the first opening in the embryo.

  • Deuterostomes: Anus develops from the first opening.

18.5 Sponges (Phylum Porifera)

Sponges are simple, sedentary animals without true tissues, characterized by a porous body and unique feeding system.

• Structure: Two layers of cells separated by a gelatinous region.

• Choanocytes: Flagellated cells that filter food from water and engulf it by phagocytosis.

• Amoebocytes: Cells that produce skeletal fibers (spicules) for support.

• Feeding: Suspension feeders; must filter large volumes of water to obtain food.

• Defense: Produce toxins and antibiotics to deter predators and pathogens.

• Example: Purple tube sponge, azure vase sponge.

18.6 Cnidarians (Phylum Cnidaria)

Cnidarians are radially symmetrical animals with tentacles and stinging cells, and have two tissue layers.

• Body Forms:

  • Polyp: Cylindrical, usually sessile (e.g., sea anemone).

  • Medusa: Mobile, bell-shaped (e.g., jellyfish).

• Feeding: Carnivorous; use tentacles with cnidocytes (stinging cells) to capture prey.

• Gastrovascular Cavity: Central digestive compartment with a single opening.

18.7 Flatworms (Phylum Platyhelminthes)

Flatworms are the simplest bilaterians, with bilateral symmetry and three tissue layers.

• Habitats: Marine, freshwater, and damp terrestrial environments.

• Groups:

  1. Free-living flatworms (Planarians): Simple brain, eyespots, branched gastrovascular cavity.

  2. Flukes: Parasitic, complex life cycles, suckers for attachment.

  3. Tapeworms: Parasitic, ribbon-like body, no mouth, absorb nutrients across body surface.

18.8 Nematodes (Phylum Nematoda)

Nematodes, or roundworms, are abundant, diverse, and have a pseudocoelom and a complete digestive tract.

• Characteristics: Bilateral symmetry, three tissue layers, nonliving cuticle, pseudocoelom, complete digestive tract (mouth and anus).

• Diversity: Estimated 500,000 species; at least 50 parasitic species infect humans.

18.9 Molluscs (Phylum Mollusca)

Molluscs are soft-bodied animals with a common body plan, including a muscular foot, visceral mass, and mantle.

• Body Plan: Muscular foot (locomotion), visceral mass (internal organs), mantle (may secrete shell), true coelom, circulatory system.

• Feeding: Many use a radula to scrape food.

• Larval Stage: Many have a ciliated trochophore larva.

• Major Groups:

  • Gastropods: Snails and slugs; largest group, often with a spiral shell.

  • Bivalves: Clams, oysters, mussels, scallops; two-part hinged shell, suspension feeders.

  • Cephalopods: Squid, octopus, nautilus; fast, intelligent predators, complex eyes, shell varies.

Additional info:

• For diagrams and figures, refer to textbook illustrations for visualizing body plans, embryonic layers, and animal phylogeny.