What are the three criteria used by scientists to classify an animal into one of the major phylum?

Scientists classify animals into major phyla based on three main criteria: body symmetry, which can be radial or bilateral; the number of tissue layers during embryonic development, distinguishing between animals with two (diploblastic) or three (triploblastic) layers; and the presence or absence of a body cavity, categorized as acoelomate, pseudocoelomate, or coelomate. These criteria help to categorize the vast diversity of animal life into systematic groups.