1. A hypothetical ancestral form of multicellular animals, consisting of a simple two-layered sac with an outer ectoderm and inner endoderm.
Usage: biology; historical; chiefly used in evolutionary biology and developmental biology contexts

Examples

• The gastrea theory proposed that all multicellular animals descended from a common two-layered ancestor.
• Early embryologists studied the gastrea as a model for understanding animal evolution.
• The concept of the gastrea influenced 19th-century thinking about the origins of body plans.