After a general view of the actual situation of the phylogenetic relationships between the diverse animals and thus having traced the fundamental elements of an essentially phylogenetic classification, several themes that emerge from this initial analysis have been developed. For instance one of the fundamental problems is about homologous genes shared by the protostomes and deuterostomes and attributed back to that hypothetical group of the primitive bilaterians, the Urbilateria, "reconstructed" from genetic analysis. The analyzed genes are essentially of a regulatory type, thus a remarkable conservation of functions seems to exist. This fundamental conservation of the general genetic patrimony of the living organisms, within an overall picture which includes an infinite gamma of structural and functional modulations of the single genes, and a vast gamma of temporal and spatial modulations of regulation of the genic activity, justifies the use of diverse animal forms for the study and comprehension of general biological phenomena, potentially relevant for human health.