Animal Diversity I: The Diversity of Invertebrates

Define characteristics that distinguish animals from other eukaryotic organisms (plants, fungi, protists).
Explore the origin and timeline of animal evolution.
Identify the four major body plan differences that classify modern animals.
Understand distinctive features of key invertebrate phyla: Porifera, Cnidaria, Mollusca, and Arthropoda.

Eukaryotic Cells: Animals are cellular organisms made of eukaryotic cells.
Multicellular Structure: They are multicellular, differentiating them from unicellular protists.
Heterotrophy: Animals obtain food by ingestion, unlike plants (autotrophs) and fungi (external digestion).
Lack of Cell Walls: Instead of cell walls, animals possess specialized structures like collagen for support and unique intercellular junctions.
Nervous and Muscle Tissue: Animals have developed nervous and muscle tissues enabling movement.

Sexual Reproduction: Most animals reproduce sexually, primarily exhibiting a dominant diploid stage.
Haploid vs. Diploid Stages:
- Haploid (n): Gametes participate in fertilization, leading to the diploid zygote.
- Diploid (2n): Represents the multicellular organisms which develop from the zygote.
Larval Stages: Many animals have larval stages that metamorphose into adults.
Hox Genes: These developmental genes regulate patterns of body organization during embryonic development.

Origin of Animals: Molecular clocks suggest the common ancestor of all animal species existed approximately 770 million years ago (MYA).
Appearance of Sponges: The earliest animal fossils, sponges, appeared around 700 MYA.
Ediacara Biota (625 MYA): Discovery of soft-bodied fossilized animals indicating early predation and ecological niches.
Cambrian Explosion (530 MYA): Dramatic rise in animal diversity, emergence of many phyla, and development of bilateral symmetry.
Causes for Diversity Surge: Possible causes include predator-prey interactions, increased atmospheric oxygen levels, and the advent of genetic factors like Hox genes.

Animal Phylogeny: Classification of animals based on criteria such as DNA, morphology, and body plans.
Body Plans Defined:
1. True Tissues: Differentiation of tissues leads to bilateral characteristics in most animals.
2. Symmetry: Evolution from radial symmetry in early forms to bilateral symmetry in more complex animals.
3. Embryonic Development: Distinctions between protostome and deuterostome development shape body plan evolution.

Characteristics: Lack true tissues; filter feeders.
Anatomy:
- Spongocoel: Central cavity for water flow.
- Choanocytes: Cells that create water currents for feeding.

Body Plan: Simple diploblastic, radial symmetry; have cnidocytes for predation.
Anatomy:
- Gastrovascular Cavity: Sac-like with one opening for digestion.
- Polyp and Medusa Forms: Two stages of life cycle.

Features: Soft-bodied organism, most with hard shells made of calcium carbonate.
Anatomy:
- Foot: Muscle used for movement.
- Visceral Mass: Contains most internal organs.
- Mantle: Secretes shells in shelled mollusks.

Diversity: Over 1 million species; crucial role in ecosystems.
Anatomy:
- Segmented bodies, jointed appendages, and exoskeleton made of chitin.
- Success Factors: Adaptations for various habitats, including land.

Characteristics of Animals:
- Multicellular, eukaryotic heterotrophs with no cell wall.
- Possess nervous and muscular tissues for movement, life cycles dominated by diploid stages, and shared larval stages.
Evolutionary Timeline: Significant diversification started around 700 MYA with distinct phyla identified through phylogenetic studies.