Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 32.3 Animals Can Be Characterized by Body Plans
Animals show extraordinary diversity, but their overall form and structure can be organized into basic body plans—combinations of morphological and developmental traits. These features guide animal classification and evolution and include symmetry, tissue layers, body cavities, and patterns of embryonic development.
Symmetry
Radial symmetry: body parts arranged around a central axis (e.g., jellies).
Suits sessile or drifting animals—interaction with environment from all sides.
Bilateral symmetry: distinct left and right sides, dorsal/ventral (top/bottom), and anterior/posterior (head/tail).
Associated with active movement and centralized nervous systems (e.g., arthropods, vertebrates).
Emerged over 550 million years ago.
Tissues and Germ Layers
Tissues: groups of specialized cells forming functional units (e.g., muscle, nerve).
Germ layers form during gastrulation and give rise to all tissues and organs:
Ectoderm: outer layer (skin, nervous system).
Endoderm: inner layer (digestive tract lining, organs).
Mesoderm (only in bilateral animals): forms muscles and other internal organs.
Diploblastic: two germ layers (e.g., cnidarians).
Triploblastic: three germ layers (e.g., most animals including vertebrates).
Body Cavities
Coelom: fluid-filled cavity lined entirely by mesoderm (e.g., earthworms).
Cushions organs, allows growth and movement independent of the body wall.
Hemocoel: cavity between mesoderm and endoderm, contains hemolymph (e.g., molluscs, arthropods).
Not a true coelom; still functional in transport.
Acoelomates: no body cavity (e.g., flatworms); rely on diffusion due to thin bodies.
Protostome vs. Deuterostome Development
These terms reflect differences in early embryonic development:
Cleavage
Protostomes: spiral, determinate cleavage (cell fates set early).
Deuterostomes: radial, indeterminate cleavage (cells remain flexible—identical twins possible).
Coelom Formation
Protostomes: forms from solid mesoderm masses that split.
Deuterostomes: forms from mesoderm outpocketings of the archenteron.
Blastopore Fate
Protostomes: blastopore becomes the mouth.
Deuterostomes: blastopore becomes the anus (mouth forms second).
In a Nutshell
Despite their diversity, animals share basic organizational features—body symmetry, tissue layers, body cavities, and embryonic development. These traits form the foundation for comparing animal phyla and tracing their evolutionary history from simple beginnings to complex structures.