Abstract: The evolutionary or ancestral history of invertebrate organisms. The animal kingdom is composed of approximately 35 phyla, with each phylum representing a distinct body plan. Only a portion of one phylum (Chordata) includes vertebrates; the remaining organisms are invertebrate animals. Traditional phylogeny is based on a number of morphological and developmental traits, or characters. The animal kingdom is probably derived from unicellular protozoanlike ancestors, which most likely underwent evolutionary processes of cytoplasmic separation to form multiple cells. It is well accepted that most animal phyla were already present in Early Cambrian times, at least 540 million years ago, and some fossil animal embryos have been found in 570-million-year-old deposits. Some highly controversial fossil interpretations have suggested animal traces that are 1 billion years in age, and some molecular studies have dated the origin of animal phyla to a similar time. However, the 1-billion-year-old fossil animal traces have been most likely misinterpreted, and molecular-clock studies used to estimate divergence times are notoriously inaccurate. The characters most often used to broadly outline invertebrate evolutionary and phylogenetic relationships include the number of embryonic tissue layers, early embryonic cleavage patterns, body symmetry, the fate of the blastopore (the opening of the cavity of the gastrula of an embryo, forming a primitive gut), the type and origin of the body cavity, and aspects of the digestive and excretory systems. In phylogenetic studies, a group of organisms (taxon) is described as monophyletic if all members evolved from an immediate common ancestor and the taxon includes all the descendants of that ancestor. A group is paraphyletic if it does not include all descendants of an immediate common ancestor, whereas it is polyphyletic if members are derived from multiple common ancestors. One important criterion for a phylum is that its members must be monophyletic