Animal Diversity Notes
🧬 General Characteristics of Animals
Animals are:
Multicellular → made of many specialized cells
Heterotrophic → must eat other organisms for energy
Eukaryotic → have a nucleus and organelles
Key Features:
Cells lack cell walls (unlike plants)
Supported by collagen (structural protein)
Have specialized tissues:
Nervous tissue → sends signals
Muscle tissue → enables movement
👉 These features allow animals to be mobile and responsive
🍽 Nutrition
Animals are ingestive heterotrophs
They consume food internally
Digest food inside their bodies
🔬 Reproduction & Development
General Pattern:
Most animals reproduce sexually
Life cycle is mostly diploid (2n)
Developmental Stages:
Fertilization → sperm + egg = zygote
Cleavage → rapid cell divisions (no growth)
Blastula → hollow ball of cells
Gastrulation → cells reorganize into layers
🧫 Germ Layers (VERY IMPORTANT)
Form during gastrulation and determine body structure:
Ectoderm (outer layer)
Skin
Nervous system
Endoderm (inner layer)
Digestive organs
Mesoderm (middle layer)
Muscles
Bones
Circulatory system
🐛 Larval Stages & Metamorphosis
Many animals have a larval stage:
Looks different from adult
Often lives in a different habitat
Metamorphosis:
Transformation into adult form
👉 Advantage: reduces competition between young and adults
🧬 Hox Genes
Control body plan development
Determine where body parts form
Highly conserved across animals
👉 Small changes in these genes → large differences in body structure
🌍 Animal Evolution
Origins:
Animals evolved ~770 million years ago
Closest living relatives:
Choanoflagellates (protists)
Important Step:
Evolution of:
Cell adhesion (cells stick together)
Cell communication
💥 Cambrian Explosion
Occurred ~535–525 million years ago
Rapid appearance of many animal groups
Possible Causes:
Increase in oxygen
Predator-prey interactions
Evolution of Hox genes
🧠 Body Plans
1. Symmetry
Radial Symmetry:
Body arranged around a central axis
No left/right sides
Example: jellyfish
Bilateral Symmetry:
Left and right sides
Has:
Anterior (head)
Posterior (tail)
Allows:
Movement in one direction
Development of a brain
🧫 Tissue Organization
Types:
No true tissues → sponges
Diploblastic → 2 layers (ectoderm + endoderm)
Triploblastic → 3 layers (includes mesoderm)
👉 All bilaterally symmetrical animals are triploblastic
🕳 Body Cavities (Coelom)
What is a Coelom?
Fluid-filled cavity between digestive tract and body wall
Functions:
Cushions organs
Allows organs to grow independently
Helps movement (acts like a skeleton)
Types:
Coelomates → true coelom
(others exist but less emphasized here)
🔄 Protostomes vs Deuterostomes (VERY IMPORTANT)
Protostomes:
Spiral cleavage
Determinate development
Cells have fixed roles early
Blastopore → mouth
Deuterostomes:
Radial cleavage
Indeterminate development
Cells can become anything (twins possible)
Blastopore → anus
🌳 Animal Phylogeny (Evolutionary Relationships)
Key Points:
All animals share a common ancestor
Sponges = earliest branch (no tissues)
Eumetazoa = animals with true tissues
Bilateria = most animals (bilateral symmetry + 3 layers)
🔺 Three Major Bilaterian Clades
1. Deuterostomia
Includes:
Echinoderms (starfish)
Chordates (vertebrates, humans)
2. Ecdysozoa
Have exoskeleton
Grow by molting (ecdysis)
Examples:
Arthropods (insects, spiders)
3. Lophotrochozoa
May have:
Lophophore (feeding structure)
Trochophore larva
Includes:
Mollusks
Annelids (worms)
🌎 Major Evolutionary Trends
Increasing complexity
Development of:
Tissues → organs → organ systems
Improved:
Movement
Feeding
Sensory systems
🧠 Big Picture Summary
Animals evolved from simple protists into complex organisms
Key innovations:
Multicellularity
Body symmetry
Germ layers
Body cavities
Modern animals are grouped based on:
Development
Body structure
Genetics