1. A hypothetical ancestral form of multicellular animals, proposed in evolutionary theory, consisting of a simple two-layered organism resembling a gastrula.
Usage: biology; historical; chiefly used in evolutionary developmental biology and comparative anatomy

Examples

• The gastraea theory suggests that modern animals evolved from a simple two-layered ancestor.
• Ernst Haeckel proposed the gastraea as a primitive form in his recapitulation theory.
• The gastraea concept has influenced modern understanding of animal body plan evolution.
• Comparative embryology was used to support the existence of a gastraea-like ancestor.