뒤로Overview of Animal Diversity: Structure, Development, and Evolution
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Ch. 32 An Overview of Animal Diversity
Learning Objectives
Describe key characteristics of animals and their life cycles.
Identify major milestones in the evolutionary history of animals.
Differentiate among types of animal body plans.
Summarize the current view of animal phylogeny.
Key Characteristics of Animals
Multicellularity and Eukaryotic Nature
Animals are multicellular eukaryotes distinguished by their ability to consume other organisms for energy and their complex cellular organization.
Multicellular eukaryotes: Animals are composed of multiple cells with a nucleus and organelles.
Ingestive heterotrophy: Animals obtain nutrients by ingesting other organisms.
Lack of cell walls: Animal cells do not have rigid cell walls, but are supported by proteins such as collagen.
Nervous and muscle tissue: These tissues are unique to animals and allow for movement and response to stimuli.
Tissues: Groups of cells with a common structure or function.
Efficient Consumption
Animals have specialized structures for feeding, digestion, and movement, making them efficient consumers in their environments.
Structures such as mouths, muscle cells, and digestive systems facilitate the breakdown and absorption of food.
Reproduction and Development
Sexual Reproduction and Life Cycle
Most animals reproduce sexually, with the diploid stage dominating the life cycle.
Fertilization: Fusion of egg and sperm to form a zygote.
Cleavage: Rapid mitotic cell divisions of the zygote, forming an eight-cell stage and then a blastula (a hollow ball of cells).
Gastrulation: The blastula undergoes rearrangement to form a gastrula, which has distinct layers of embryonic tissues.
Developmental sequence:
Egg + Sperm → Fertilization → Cleavage → Blastula → Gastrula (zygote)
Embryonic Tissue Layers
Ectoderm: Outermost germ layer covering the embryo's surface.
Endoderm: Innermost germ layer lining the digestive tract.
Mesoderm: Intermediate tissue layer present in triploblastic animals.
Larval Stages and Metamorphosis
Many animals have a larval stage, which is sexually immature and morphologically distinct from the adult. The process of transformation from larva to adult is called metamorphosis.
Juvenile: Resembles the adult but is not yet sexually mature.
Animal Development
Genetic Control of Development
Animal development is regulated by specific genes, including homeoboxes and Hox genes.
Homeoboxes: DNA sequences that regulate the expression of other genes during embryonic development.
Hox genes: A subset of homeobox genes that control the body plan and segmentation in animals.
These genes influence morphology, segmentation, and the development of appendages.
Evolutionary History of Animals
Major Milestones
The history of animals spans more than half a billion years, with significant evolutionary events shaping their diversity.
Cambrian explosion: Rapid diversification of animal forms about 541 million years ago.
Common ancestor: The ancestor of living animals likely existed between 675 and 800 million years ago.
Origin of Animals
The common ancestor of animals likely resembled modern choanoflagellates, unicellular or colonial protists with a collar of microvilli.
Evidence from molecular and morphological studies supports this relationship.
Animal Body Plans
Definition and Components
A body plan is a set of morphological and developmental traits integrated into a functional whole organism.
Symmetry
Tissues
Body cavities
Developmental patterns
Body Symmetry
Animals can be classified based on their symmetry:
Radial symmetry: Body parts arranged around a central axis; typical of sessile or planktonic animals (e.g., jellyfish).
Bilateral symmetry: Two-sided symmetry with distinct dorsal (top), ventral (bottom), anterior (head), and posterior (tail) ends. Associated with cephalization (development of a head).
Tissues and Germ Layers
Animals are classified by the number of germ layers formed during development:
Diploblastic: Animals with two germ layers (ectoderm and endoderm), such as cnidarians.
Triploblastic: Animals with three germ layers (ectoderm, mesoderm, endoderm); all bilaterally symmetrical animals are triploblastic.
Summary Table: Animal Body Plan Features
Feature | Radial Symmetry | Bilateral Symmetry |
|---|---|---|
Body Axis | Central axis | Dorsal, ventral, anterior, posterior |
Movement | Usually sessile or planktonic | Active movement, cephalization |
Germ Layers | Diploblastic | Triploblastic |
Examples | Jellyfish, sea anemones | Vertebrates, insects |
Key Terms and Definitions
Multicellular eukaryotes: Organisms composed of multiple cells with a nucleus.
Heterotrophy: Obtaining energy by consuming other organisms.
Collagen: Structural protein in animal cells.
Cleavage: Rapid cell division after fertilization.
Blastula: Hollow ball of cells formed after cleavage.
Gastrula: Embryonic stage with multiple tissue layers.
Homeobox: DNA sequence regulating development.
Hox genes: Genes controlling body plan and segmentation.
Cephalization: Development of a head region.
Diploblastic: Two germ layers.
Triploblastic: Three germ layers.
Relevant Equations
Sequence of animal development:
Additional info:
Some context and definitions have been expanded for clarity and completeness.
