Noun
The word is derived from metazoan
Those animals in which the protoplasmic mass, constituting the egg, is converted into a multitude of cells, which are metamorphosed into the tissues of the body. A central cavity is commonly developed, and the cells around it are at first arranged in two layers, -- the ectoderm and endoderm. The group comprises nearly all animals except the Protozoa.
Source: Webster's dictionaryHowever, another comparison in 2008 of 150 genes in each of 21 genera, ranging from fungi to humans but including only two species of sponge, suggested that comb jellies ( ctenophora ) are the most basal lineage of the Metazoa included in the sample. Source: Internet
If this is correct, either modern comb jellies developed their complex structures independently of other Metazoa, or sponges' ancestors were more complex and all known sponges are drastically simplified forms. Source: Internet