Regnum Animalia

Using the five-kingdom system of classification (Margulis and Schwartz, 1998), members of the Kingdom Animalia are defined as heterotrophic, diploid multicellular organisms that usually develop from a blastula. The blastula is a multicellular embryo unique to the Animalia. It develops by mitotic cell divisions from a diploid zygote, formed by fertilisation of a large haploid egg by a smaller haploid sperm. These haploid gametes are formed within diploid adults by meiosis.

Of the multicellular organisms, animals are the most structurally complex; cells with highly specialized functions are grouped into tissues and tissues into organs. Form and behaviour are also most diverse in the Animalia. Nutrition is normally by ingestion.